THE 
 
 ELEMENTS OF. EMBRYOLOGY. 
 
 BY 
 
 M. FOSTER, M.A., M.D., F.R.S., 
 
 FELLOW OF AND PRJELECTOR IN PHYSIOLOGY IN TRINITY COLLEGE, CAMBRIDGE, 
 
 AND 
 
 FRANCIS M. BALFOUR, B.A., 
 
 FEUOW OF TRINITY COLLEGE, CAMBRIDGE. 
 
 UonUon : 
 
 MACMILLAN AND CO. 
 1874. 
 
 [All Eights reserved.] 
 
F4 
 
 OGY 
 LlfcBART 
 
 a 
 
 Camfcrftge : 
 
 PRINTED BY C. J. CLAY, M.A. 
 
 AT THE UNIVERSITY PRESS. 
 
 
TO 
 
 THOMAS HENEY HUXLEY 
 
 AS A LITTLE TOKEN 
 OF OUR APPRECIATION OF HIS WORTH 
 
 AND OF 
 HIS MUCH KINDNESS TO OURSELVES 
 
 THIS BOOK 
 
 IS RESPECTFULLY DEDICATED BY 
 
 ' E ATJTHOKS. 
 
 
 
PKEFACE. 
 
 IN this volume we offer to the public the first part of what 
 we hope may serve as a systematic introduction to the study 
 of Embryology. Some apology is perhaps necessary for the 
 separate publication of a part only of the whole subject; but 
 we trust that the following reasons will justify the course we 
 have adopted. 
 
 Those who have paid attention to recent embryological 
 researches must be aware of what we may venture to call 
 the tumultuous condition of many parts of the subject, and 
 of the extreme difficulty in many cases of forming a clear 
 and decided judgment without the aid of independent 
 observations. It is this necessity of having repeatedly to 
 work over contested points with a view to reconcile 
 diametrically opposed .statements, or to verify startling 
 announcements, which has rendered so laborious the task 
 we have undertaken, and which so much delays its com- 
 pletion. 
 
 On the other hand, whoever wishes to have a sound 
 foundation of embryological knowledge cannot do better 
 than gain a thorough insight into the development of the 
 bird. The practical advantages offered by the hen's egg 
 
 62 
 
viii PREFACE. 
 
 altogether outweigh the theoretical objections to beginning 
 with the avian type. In many respects, it might be thought 
 desirable to commence with a holoblastic ovum ; but the 
 large food-yolk of the bird's egg is in many ways a great 
 assistance to the study of changes going on in the blasto- 
 derm. The chick is of all embryos the best to begin with ; 
 when its history has once been mastered, the subsequent 
 study of other forms becomes an easy matter. 
 
 We venture to hope therefore that we shall meet with 
 general approval, in having described at considerable length 
 the history of the chick, and in hastening the publication of 
 our account, by bringing it forward in a separate form. 
 
 In the earlier chapters, especially, we have gone into 
 very considerable detail; and in order to make the account 
 intelligible to the beginner, have not been deterred by the 
 fear of wearying our readers with elementary and recapitu- 
 latory statements. Debated matters and details of minor 
 importance have been put in small print ; these may be 
 omitted by the student in reading the book for the first 
 time. Though we have sometimes introduced names in 
 connection with important observations, we have not thought 
 it necessary to do this systematically. For recent or debated 
 statements however, the authorities .are always cited. 
 
 The worth of such a book as this will be very small if 
 the student simply contents himself with reading what is 
 written; and to facilitate the only really useful mode of 
 study, that of actual observation, a few practical instructions 
 have been added in an appendix. 
 
 The readiness with which the development of the skull 
 can be studied in the chick renders it, in spite of obvious 
 
PREFACE. ix 
 
 objections, a suitable introduction to the important subject 
 of cranial morphology. It is with this view that we have 
 given a separate chapter on the skull, which we hope may 
 serve as an introduction to the study of Mr Parker's elaborate 
 memoirs. 
 
 In the remaining parts, which we shall do our best to 
 complete as soon as possible, the several histories will be 
 treated with much greater brevity, and much more space will 
 be given to theoretical considerations. 
 
 The figures, whose source is not acknowledged in the 
 text, were drawn by Miss A. B. Balfour, except a few by 
 ourselves. 
 
 The drawing on wood was executed partly by Mr Allchin, 
 but chiefly by Mr Collings ; and all the drawings were cut 
 by Mr Cooper. We have to thank those gentlemen for the 
 trouble they have taken in a matter in which, for many 
 reasons, the result never seems commensurate with the 
 labour. We are much indebted to Professor Huxley for 
 having kindly looked over the proofs of the Chapter on the 
 Skull. 
 
 The work took its origin in a course of lectures delivered 
 by myself, but many causes prevented my taking the task 
 seriously in hand, until I was joined by my friend and 
 former pupil Mr F. M. Balfour, whose share in the matter 
 has, to say the least, been no less than my own. 
 
 M. FOSTEK. 
 
TABLE OF CONTENTS. 
 
 INTRODUCTION ... , pp. i 10. 
 
 PART I. THE HISTORY OF THE CHICK. 
 
 CHAPTER I. 
 
 THE STRUCTURE OP THE HEN'S EGG, AND THE CHANGES WHICH TAKE PLACE 
 UP TO THE BEGINNING OP INCUBATION . . pp. II 26. 
 
 i. The shell, i. The shell-membrane. 3. The albumen. 4. The vitelline 
 membrane. 5. The yolk. 6. The yellow yolk. 7. The white yolk. 8. The 
 white yolk-spheres. 9. The structure of the blastoderm. 10. Recapitulation, 
 n. The ovarian ovum. 12. The descent of the ovum along the oviduct. 
 13. The impregnation of the ovum. 14. Segmentation. 15. The formation of 
 the upper and lower layers. 
 
 CHAPTER II. 
 
 A BRIEF SUMMARY OP THE WHOLE HISTORY OP INCUBATION, pp. 2742. 
 
 i. The embryo is formed in the area pellucida. -2. The epiblast, mesoblast, 
 and hypoblast. 3. The extension of the blastoderm over the yolk. 4. The 
 vascular area. 5. The head-fold and the other folds by means of which the 
 embryonic sac is formed. 6. The outward shape of the embryo. 7. The 
 formation of the neural tube and alimentary canal: somatopleure and splanch- 
 iiopleure. 8. The amnion. 9. The allantois. 
 
Xll CONTENTS. 
 
 CHAPTER III. 
 
 THE CHANGES WHICH TAKE PLACE DURING THE FIRST DAY OF INCUBATION, 
 
 PP- 4357- 
 
 I. Variations in the progress of development. 2. The embryonic shield. 
 3. The formation of the epiblast, mesoblast and hypoblast. 4. The primitive 
 streak, the primitive groove. 5. The head-fold, the medullary groove, me- 
 dullary folds, and notochord. 6. The amnion ; the changes taking place in the 
 three layers. 7. The increase of the head-fold. 8. The closure of the me- 
 dullary canal. 9, 10. The cleavage of the mesoblast : formation of splanchno- 
 pleure and somatopleure. ir. The protovertebrse. 12. The formation of the 
 vascular area. 13. Recapitulation. 
 
 CHAPTER IV. 
 
 THE CHANGES WHICH TAKE PLACE DURING THE SECOND DAY, pp. 58 83. 
 
 I. Increasing distinctness and prominence of the embryo. 2. The first cere- 
 bral vesicle. 3. The increase in the number of proto vertebrae. 4. The first 
 rudiments of the alimentary canal. 5. The formation of the heart. 6. The 
 formation of blood-vessels ; the omphalo-mesaraic veins and arteries, the sinus 
 terminalis. 7. Changes taking place in the cells of the several layers. 
 8. The rudiment of the Wolffian duct. 9. Recapitulation of the changes 
 during the first half of the second day. 10. Increasing prominence of the 
 embryo ; the tail-fold and the lateral folds. 1 1. Continued closure of the 
 medullary canal. 12. The optic vesicles. 13. The second and third cerebral 
 vesicles. 14. Change of position of the optic vesicles. 15. The vesicles of the 
 cerebral hemispheres. 16. The cranial flexure. 17. The rudiment of the ear, or 
 auditory sac. 18. Changes in the heart. 19. The primitive aortse and first 
 pair of aortic arches, the omphalo-mesaraic vessels, the sinus terminalis. 
 20. The second and third pair of aortic arches. 11. The Wolffian duct. 
 22. The amnion. 23. Recapitulation. 
 
 CHAPTER V. 
 
 THE CHANGES WHICH TAKE PLACE DURING THE THIRD DAY, pp. 84 140. 
 
 I. The diminution of the albumen. 2. The spreading of the opaque and 
 vascular areas. 3. The vascular area. 4. The continued folding in of the 
 embryo. 5. The increase of the amnion. 6. The change in the position of 
 the embryo. 7. The curvature of the body. 8. The cranial flexure. 
 9* Growth of the vesicles of the cerebral hemispheres; the third ventricle, 
 pineal gland, infundibulum and pituitary body, the cerebellum and medulla 
 oblongata. 10. Changes in the spinal cord. n. The formation of the eye. 
 Histological changes in the retina, optic nerve, and lens. 12. The formation 
 of the ear. 13. The nasal pits* 14. The visceral clefts and folds. 15. The 
 aortic arches. 16. Changes in the heart; the Ductus Cuvieri and cardinal 
 veins. 17. The folding in of the alimentary canal; the formation of the tail. 
 1 8. The lungs. 19. The liver. 20. The pancreas and spleen. 21. The 
 thyroid body. 22. Changes in the trunk of the embryo. 23. Separation of 
 the muscle-plates from the protovertebrse. 24. Growth of the intermediate 
 cell-mass. 25. The cranial nerves. 26. The Wolffian duct. 27. Recapi- 
 tulation. 
 
CONTENTS. xiil 
 
 CHAPTER VI. 
 
 THE CHANGES WHICH TAKE PLACE DURING THE FOURTH DAY, pp. 141 173. 
 
 j. Appearances on opening the egg. 2. Growth of the amnion. 
 3. Narrowing of the splanchnic stalk. 4. Increase in the cranial flexure. 
 
 5. The first appearance of the limbs. 6. Growth of the head. 7. Changes 
 in the nasal pits. 8. Formation of the mouth. Q. The cranial nerves. 
 10. .The allantois. n. Changes in the protovertebras ; the spinal ganglia. 
 12. The secondary segmentation of the vertebral column. 13. Changes in 
 the notochord. 14. Ossification of the vertebrae. 15. The ribs. if>. 
 Changes in the muscle-plates. 17. The Wolffian body and duct. 18. The 
 duct of Mu'ller. 19. The kidneys. 20. The ovaries and testes. i\. 
 Changes in the arterial system. 22. Changes in the venous system; the 
 veins of the liver. 23. Changes in the heart; the ventricular septum. 
 24. Recapitulation. 
 
 CHAPTER VII. 
 THE CHANGES WHICH TAKE PLACE ON THE FIFTH DAY, pp. 174 199. 
 
 i. Appearances on opening the egg. i. The growth of the limbs. 
 3. The cranium; the investing mass and trabeculae. 4. Changes in the 
 face ; formation of the nose and nasal passages. 5. Appearance of the anus. 
 
 6. Changes in the spinal cord ; the formation of the grey and white columns, 
 and of the posterior and anterior fissures. 7. Changes in the heart; the 
 rudiment of the auricular septum, the division of the bulbus arteriosus into 
 aorta and pulmonary artery, the formation of the semilunar valves. 8. Changes 
 in the heart during the sixth day. 9. Subsequent changes in the heart; the 
 completion of the auricular septum, the arrangement of the openings of the 
 venae cavte. 10. Histological differentiation; the fate of the three primary 
 layers, u. Recapitulation. 
 
 CHAPTER VIII. 
 
 FKOM THE SIXTH DAY TO THE END OF INCUBATION, pp. 200 224. 
 
 r. The commencement of distinct avian differentiation. 2. The foetal 
 appendages during the sixth and seventh days. 3. During the eighth, ninth 
 and tenth days. 4. From the eleventh to the sixteenth day. 5. From the 
 sixteenth day onwards. 6. The changes in the general form of the embryo 
 during the sixth and seventh days. 7. During the eighth, ninth and tenth 
 days. 8. From the eleventh day onwards ; feathers, ossifications. 9. Changes 
 in the venous system before and after the commencement of pulmonary respi- 
 ration. 10. Changes in the arterial system, the modifications of the aortic 
 arches, u. Summary of the chief phases of the circulation. 12. Ex- 
 clusion from the egg. 
 
xiv CONTENTS. 
 
 CHAPTER IX. 
 
 THE DEVELOPMENT OF THE SKULL, pp. 225 238. 
 
 i, 2. The primordial cranium. 3, 4. The investing mass of Rathke. 
 5. The trabeculse cranii. 6. The cartilages of the first visceral arch. 7. 
 The maxillary process. 8. The mandibular arch. 9. The hyoid arch. 10. 
 The cartilages of the third visceral arch. 1 1. Changes in the cranium during 
 the fifth and sixth days. 12. During and after the seventh day. 13. The 
 condition of the cranium at about the middle of the second week. 14. Ecto- 
 steal and endosteal ossifications of the cartilaginous cranium. 15. Formation 
 of the membrane bones. 16. Progress of ossification during the second and 
 third weeks. 17. Fenestration of the ethmo-presphenoid cartilage. 18. Ossifi- 
 cations in the prootics and alisphenoid. 19. Changes in the basitemporals. 
 Formation of the vomer. 20. The changes which take place immediately 
 after exclusion from the egg. 21. Further changes in the splint bones. 
 Coalescence of the bones after birth. Table of bones classified according to 
 their mode of ossification. 
 
 APPENDIX. 
 
 PEACTICAL INSTRUCTIONS FOE STUDYING THE DEVELOPMENT OP THE CHICK, 
 pp. 239267. 
 
 I. Incubators. II. Examination of a 36 to 48 hours embryo. III. 
 Examination of an embryo of about 48 50 hours. IV. Of an embryo at the 
 end of the third day. V. Of an embryo of the fourth day. VI. Of a 
 blastoderm of 20 hours. VII. Of an unincubated blastoderm. VIII. Of the 
 process of segmentation. IX. Of the later changes of the embryo. X. Study 
 of the development of the blood-vessels. 
 
 ERRATUM. 
 
 p. 124, in the description of Fig. 39 B, for 'Superior vertebral' substitute 
 'Jugular.' 
 
LIST OF ILLUSTKATIONS. 
 
 FIG. PAGE 
 
 1. DIAGRAMMATIC Section of an Un incubated Fowl's Egg . . .12 
 
 2. A. Yellow yolk-sphere filled with fine granules. J5. White yolk- 
 
 spheres and ^spherules of various sizes and presenting different ap- 
 pearances '... " ' , * V , . . . . . 15 
 
 3. Section of a Blastoderm of a Fowl's Egg at the commencement of 
 
 Incubation * * . . . . . . 18 
 
 4. Section througli the Germinal Disc of the ripe Ovarian Ovum of a 
 
 Fowl while yet enclosed in its Capsule . . . . . 19 
 
 5. Surface Views of the early Stages of the Segmentation in a Fowl's 
 
 Egg . . , . . .22 
 
 6. Surface View of the Germinal Disc of a Hen's Egg during the later 
 
 Stages of Segmentation . . . ; . . . 23 
 
 7. Section of the Germinal Disc of a Fowl during the later Stages of 
 
 Segmentation . . . . . , '. : . . . 24 
 
 8. A to N. A series of purely diagrammatic representations introduced 
 
 to facilitate the comprehension of the manner in which the body of 
 the embryo is formed, and of the various relations of the yolk-sac, 
 amnion and allantois ........ 29 32 
 
 9. Diagrammatic Longitudinal Section through the Axis of an Embryo. 33 
 
 10. Section of a Blastoderm at right angles to the long axis of the 
 
 Embryo after eight hours' Incubation ...... 46 
 
XV i LIST OF ILLUSTRATIONS. 
 
 FIG. 
 
 11. Surface View of the Pellucid Area of a Blastoderm of 18 hours . 49 
 
 12. Transverse Section of a Blastoderm incubated for 1 8 hours . . 51 
 
 13. Transverse Section through the Dorsal Region of an Embryo of the 
 
 Second Day , . : , . V . * ... V, . 55 
 
 14. An Embryo Chick of the First Day (about thirty-six hours) viewed 
 
 from below as a transparent object . '. '. ' A , . ... 59 
 
 15. Embryo of the Chick at 36 hours viewed from above as an opaque 
 
 object . . ..''.. 6 1 
 
 1 6. Diagrammatic Longitudinal Section through the Axis of an Embryo. 62 
 
 17. A, B. Two consecutive Sections of a 36 hours Embryo illustrating 
 
 the formation of the heart . . . . . . . .67 
 
 1 8. Transverse Section of an Embryo at the end of the Second Day 
 
 passing through the region of bulbus arteriosus .... 68 
 
 19. Surface View from below of a small portion of the posterior end of 
 
 the pellucid area of a 36 hours Chick * . . . . 70 
 
 20. Transverse Section through the Dorsal Region of an Embryo of 45 
 
 hours ............ 74 
 
 21. Embryo of the Chick at 36 hours viewed from above as an Opaque 
 
 Object , . ' ' 75 
 
 22. Head of a Chick at the End of the Second Day viewed from below 
 
 as a Transparent Object . . . . . . . .76 
 
 23. Diagram of the Circulation of the Yolk-Sac at the end^of the Third 
 
 Day of Incubation . . . . . ... .85 
 
 24. Chick of the Third Day (54 hours) viewed from underneath as a 
 
 Transparent Object i . . . 88 
 
 25. Head of a Chick of the Third Day viewed sideways as a Transparent 
 
 Object . ' s . . . . .... . 90 
 
 26. Section through the Hind-Brain of a Chick at the end of the Third 
 
 Day of Incubation . . . . . . * : 'i .-... . * jr 95 
 
 27. Diagrammatic Sections illustrating the Formation of the Eye . . 97 
 
 28. Diagrammatic Section of the Eye and the Optic Nerve at an early 
 
 .stage .... ' ....,', *';*''.;. ,' .'. ' ,f V ,t . 4* 98 
 
LIST OF ILLUSTRATIONS. XV11 
 
 FIG. PAGE 
 
 29. Diagrammatic representation of the Eye of the Chick of about the 
 
 Third Day as <seen when the head is viewed from underneath as a 
 transparent object .... f .... 99 
 
 30. D, E, F. Diagrammatic Sections of the Eye of the Chick of about 
 
 the Third Day . , " , . ; , . . . ; . . . . 100 
 
 31. Section of the Eye of Chick at the Fourth Day . ,-.' . * ':. . 104 
 
 32. Section through the Hind-Brain of a Chick at the end of the Third 
 
 Day of Incubation . . . . , . . . . no 
 
 33* Two Views of the membranous Labyrinth of Columba domestica. 
 
 A from the exterior, B from the interior ,,. * . . 112 
 
 34. Transverse Section of the Head of a Foetal Sheep (16 mm. in 
 
 length) in the region of the Hind-Brain . . t , 113 
 
 35. Section of the Head of a Foetal Sheep (20 mm. in length) . .114 
 
 36. Section through "the internal Ear of an Embryonic Sheep (28 mm. 
 
 in length) . ,. 'f, .,. .' . . . 116 
 
 37. Head of an Embryo Chick of the Third Day viewed sideways as an 
 
 Opaque Object . . . . . , . * ; - >; /., nS 
 
 38. The same Head as shewn in Fig. 37, seen from the Front . 121 
 39 A. Diagram of the Arterial Circulation on the Third Day . . 122 
 39 B. Diagram of the Venous Circulation on the Third Day '', ; . 124 
 
 40. Section of the Tail-end of an Embryo (Chick) of the Third Day . 125 
 
 41. Section through the Dorsal Ilegion of an Embryo at the commence- 
 
 ment of the Third Day ' ..*'* * "V, "*- * 126 
 
 42. Diagram of a portion of the Digestive Tract of a Chick upon the 
 
 Fourth Day. . ' . ..".".* ., , . . i^S 
 
 43. Four diagrams illustrating the Formation of the Lungs . . . 129 
 
 44. Section through the Dorsal Region of an Embryo at the end of 
 
 the Third Day . v . . ' , ~, >, . ."-\ . . 135 
 
 45. Head of an Embryo Chick of the Third Day (seventy-five hours) 
 
 viewed sideways as a transparent object : ' . . 137 
 
 46. Embryo at the end of the Fourth Day seen as a transparent 
 
 .object. . . . . . . . . . . .142 
 
XV111 LIST OF ILLUSTRATIONS. 
 
 FIG. PAGE 
 
 47. Section through the Lumbar Region of an Embryo at the End of 
 
 the Fourth Day 144 
 
 48. A. Head of an Embryo Chick of the Fourth Day viewed from 
 
 below as an opaque object. B. The same seen sideways . .146 
 
 49. Longitudinal Section of the Tail-end of an Embryo Chick at the 
 
 commencement of the Third Day . . . . . .148 
 
 50. Longitudinal Section of the Tail-end of an Embryo Chick at the 
 
 middle of the Third Day .* - . . . . . . . 149 
 
 51. Section of the intermediate Cell-mass on the Fourth Day . .165 
 
 52. State of Arterial Circulation on the Fifth or Sixth Day .">'' 169 
 
 53. Diagram of the Venous Circulation at the Commencement of the 
 
 Fifth Day .''.,, . * , > . . ' * . 170 
 
 54. Heart of a Chick on the Fourth Day of Incubation viewed from 
 
 the Ventral Surface . * > ( * . . N * . 172 
 
 55. View from above of the Investing Mass and of the Trabeculae on 
 
 the Fourth Day of Incubation > *-. * . , . 177 
 
 e,6. A. Head of an Embryo Chick of the Fourth Day viewed from 
 
 below as an opaque object. B. The same seen sideways. . 180 
 
 57. Head of a Chick at the Sixth Day from below '* . . . . 181 
 
 58. Head of a Chick of the Seventh Day from below ... ... . 182 
 
 59. Section through the Spinal Cord of a Seven Days Chick . . 188 
 
 60. Two views of the Heart of a Chick upon the Fifth Day of Incuba- 
 
 tion . . .4 . V ( > " . ' .192 
 
 61. Heart of a Chick upon the Sixth Day of Incubation, from the 
 
 Ventral Surface . ,*'*-' . i \ . . 193 
 
 62. Diagram of the Venous Circulation at the Commencement of the 
 
 Fifth Day . . . < . '.. . * . . 206 
 
 63. Diagram of the Venous Circulation during the later days of Incu- 
 
 bation . . m * *" V % . . aoS 
 
 64. Diagram of the Venous Circulation of the Chick after the com- 
 
 mencement of Respiration by means of the Lungs . . .n 
 
 65. State of Arterial Circulation on the Fifth or Sixth Day . . . ii2 
 
LIST OF ILLUSTRATIONS. xix 
 
 FIG. PAGE 
 
 66. Diagram of the Condition of the Arches of the Aorta towards the 
 
 Close of Incubation . 216 
 
 67. Diagram of the Arterial System of the Adult Fowl . . .219 
 
 68. View from above of the Investing Mass and of the Trabeculse on 
 
 the Fourth Day of Incubation . . . . . . .226 
 
 69. View from below of the Paired Appendages of the Skull of a Fowl 
 
 on the Fourth Day of Incubation .,*.',.. . . 229 
 
 70. Side view of the Cartilaginous Cranium of a Fowl on the Seventh 
 
 Day of Incubation * . * ,. . . . . . 231 
 
 71. Embryonic Skull of a Fowl during the Second Week of Incubation 
 
 (third stage) from below . . . . . . . .234 
 
INTRODUCTION. 
 
 EVERY living being passes in the course of its life through a 
 series of changes of shape and structure. These changes 
 may, in their completest form, be considered as constituting 
 a morphological cycle, beginning with the ovum and ending 
 with the ovum again. 
 
 Among many living beings and especially among verte- 
 brate animals by far by the greater part of the life of the 
 individual is spent in one particular phase, which is not only 
 of longer duration than the rest, but also of much more 
 importance, inasmuch as during it the greater part of the 
 ' work ' of the living being is done. This is generally spoken 
 of as the adult stage, and in most cases immediately precedes, 
 or is peculiarly associated with, the completion of the morpho- 
 logical cycle in the appearance of a new ovum. 
 
 The word embryology may be generally taken to mean 
 the study of the successive morphological phases through 
 which a living being passes from the ovum to the adult 
 stage, or the study of the gradual 'development' of the ovum 
 to the adult form ; though, especially among some of the so- 
 called lower forms of life, its meaning must be so extended 
 as to embrace all the morphological phases of an individual 
 life. Embryology is thus a part of and a necessary intro- 
 duction to the wider study of ' Generation.' As a matter of 
 
 / E. 1 
 
2 f V t Ctf EM^YOLOGY. 
 
 history w^ad'f-Kat {tTie.^udy ,of it Sprang out of the various 
 attempts to solve the problems of why and how living beings 
 come into existence. 
 
 It would be beyond the scope of this work to enter at 
 all fully into any account of the earlier of these inquiries 
 from those of Aristotle downwards ; but it may be of some 
 use to point out the chief steps by which in modern times 
 embryology has been established as a distinct branch of 
 knowledge. 
 
 From the very first, incubated bird's eggs, and especially 
 hen's eggs, owing to their abundance at all seasons, and the 
 ease with which they could be examined, became special 
 objects of study. Aristotle examined the growing chick 
 within, the egg, and gave the name of punctum saliens to the 
 ' bloody palpitating point/ which marks the growing heart in 
 the early days of incubation. Since his time all observers 
 have had recourse to the hen's egg; and though it may be 
 urged that the highly specialised characters of the avian 
 type unfit it for so general a purpose as that of serving as 
 the foundation of embryology, the practical advantages of the 
 bird's egg over either the mammalian or any other ovum, are 
 so many, that it must always continue to be, as it has been, 
 a chief object of study. 
 
 From the time of Aristotle down to that of Fabricius of 
 Aquapendente so little progress in real observation of facts 
 had been made, that we find the latter anatomist (De Forma- 
 tione Ovi et Pulli, 1 621) describing the chick as being formed 
 out of the chalazae of the white of the egg; a view which 
 lived long afterwards, and whose influence may still be 
 recognized in the names 'tread' or 'treadle' which the 
 housewife sometimes gives to those portions of thickened 
 albumen. 
 
 Harvey was the first to clearly establish that the essential 
 part of the hen's egg, that out of which the embryo pro- 
 
INTRODUCTION. 3 
 
 ceeded, was the cicatricula. This Fabricius had looked upon 
 as a blemish, a scar left by a broken peduncle. In his 
 Anatomical Exercises on the Generation of Animals (1651), 
 Harvey describes the little cicatricula as expanding under 
 the influence of incubation into a wider structure, which 
 he calls the eye of the egg ; and at the same time sepa- 
 rating into a colliquamentum. In this colliquamentum, 
 according to him, there appears, as the first rudiment of 
 the embryo, the heart or punctum saliens, together with 
 the blood-vessels. These gradually gather round them 
 the solid parts of the body of the chick. Harvey clearly 
 was of opinion that the embryo arose, by the successive 
 formation of parts, out of the homogeneous nearly liquid 
 colliquamentum. He was an early advocate of the doctrine 
 of epigenesis. 
 
 Notwithstanding the weight of Harvey's authority, the 
 doctrine of epigenesis subsequently gave way to that of 
 evolution, according to which the embryo pre-existed, even 
 though invisible, in the ovum, and the changes which took 
 place during incubation consisted not in a formation of 
 parts, but in a growth, i. e. in an expansion with concomitant 
 changes, of the already existing germ. Of this theory 
 Malpighi is frequently said to have been the founder. In 
 a limited sense this is true. In his letter to the Royal 
 Society of London, De Formatione Pulli in Ovo (1672), he 
 
 :onfesses himself compelled to admit that even in unincu- 
 bated eggs an embryo was present (Quare pulli stamina in 
 ovo pre-existere, altioremque originem nacta esse fateri 
 convenit). Yet he evidently struggled against such a con- 
 clusion, and instead of developing a consistent theory of 
 
 evolution, left the earliest stages of the embryo as too 
 mysterious to be profitable objects of study, and contented 
 himself with tracing out the events of later days. From his 
 descriptions it is clear that his so-called unincubated eggs 
 
 12 
 
4 ON EMBRYOLOGY. 
 
 had under the warmth of summer already made considerable 
 progress in development. 
 
 The man who first logically worked out a theory of 
 evolution and became its most distinguished and zealous 
 advocate was Haller (Sur la Formation du Cceur dans le 
 Poulet, 1758, arid Elementa Physiologic, Liber xxix. 1766). 
 
 This great anatomist insisted that the embryo existed 
 even in the unincubated egg though in a rudimentary form, 
 and indeed invisible. He supposed that it was a vermiform 
 structure composed of all the essential parts of a full-grown 
 animal in an undeveloped state, and that the effect of incu- 
 bation was to educe or evolve these undeveloped organs into 
 an adult condition. The same views were urged with cha- 
 racteristic extravagance by Bonnet (Considerations sur les 
 corps organises, 1762). 
 
 This doctrine of evolution or prsedilineation, as it was 
 called at the time, was doomed to be overthrown even in 
 Haller 's own day. 
 
 In an inaugural dissertation entitled Theoria Generationis, 
 published 1759, Casper Frederick Wolff laid the foundations 
 of not only modern Embryology, but modern Histology. He 
 shewed that the cicatricula of the unincubated hen's egg con- 
 sisted of a congeries of particles (such as we now call cells) all 
 alike, or divisible into groups only, and that anything like 
 distinct rudiments of an embryo were wholly absent. Out of] 
 these particles the embryo was built up by means of a series of j 
 successive changes (several of which he described in detail, 
 especially in his work on the Formation of the Alimentary 
 Canal, 1768), part being added to part, and parts once formed 
 being modified into fresh parts. Thus the old imperfect 
 theory of evolution was supplanted by a view, which, under 
 the term of epigenesis, was in reality a more complete and 
 truer theory of evolution. Wolff also shewed that all the, 
 parts as well of plants as of animals could be conceived of 
 
INTRODUCTION. 5 
 
 as being arrangements of these particles or cells variously 
 modified, and that all the phenomena of the form and 
 structure of living beings were to be regarded as the results 
 of a variable nutritive energy, to which he gave the name 
 vis essentialis. 
 
 Haller complained of Wolff, that he had attempted to 
 make a great leap instead of being contented with small on- 
 ward steps. Wolff's leap proved too great for his time. While 
 his insight into the fundamental doctrines of histology re- 
 mained for the most part without fruit till the next century, 
 so also the way he opened up in embryology was successfully 
 followed by no one for many years after. 
 
 In 1816 that admirable teacher Dollinger, of Wiirzburg, 
 induced Pander to take up the study of the incubated hen's 
 egg. We owe to Pander (Dissertatio Inauguralis sistens 
 Historiam Metamorphoseos quam Ovum Incubatum prioribus 
 quinque diebus subit, and Beitrdge zur Entwickelungsge- 
 schichte des Huhnchens im Eie) a clear and excellent descrip- 
 tion of many of the changes which take place during the 
 early days of incubation. It was he who introduced the term 
 blastoderm. He too first drew attention to the distinction of 
 the three layers, serous, mucous, and vascular. But his 
 greatest merit perhaps consisted in the fact of his studies 
 having been the exciting cause of those of Von Baer. 
 
 Coming to Wiirzburg to study under Dollinger, and finding 
 Pander busily engaged in his embryological work, Von Baer 
 enthusiastically took up the same subject, and thenceforward 
 devoted the greater part of his life to it. 
 
 Of the results of his labours, which are embodied in his 
 Entwickelungsgeschichte der Thiere, 1828, 1837, this simply 
 may be said. Von Baer found the true line of inquiry already 
 marked out by Wolff. He followed up that line so sedulously 
 and with such success, that nearly all the work which has 
 been done since his day up to the present time, in Vertebrate 
 
6 ON EMBRYOLOGY* 
 
 Embryology, may be regarded as little more than an ex- 
 tension, with corrections, of his observations. Were it de- 
 sirable to re-publish Von Baer's work, the corrections and 
 expansions of matters of fact necessary to bring it up to the 
 present time, as the phrase goes, would, with some few 
 exceptions, be of minor importance, though they might be 
 many. The theoretical considerations embodied in his 
 Scholia through which he interprets the morphological sig- 
 nificance of embryological facts are of great and lasting 
 importance, though they need some modifications in order to 
 bring them into harmony with the theory of natural selection. 
 Since Von Baer's time, the advances made in Vertebrate Em- 
 bryology, through the elaborate work of Remak, the labours of 
 Eathke, Allen Thomson and others, the admirable lectures of 
 Kolliker, and the researches of more recent inquirers, though 
 many and varied, cannot be said to constitute any epochs in 
 the history of the subject, such as that which was marked by 
 Von Baer, and before him by Wolff. We may perhaps make 
 an exception in favour of the discovery by Purkinje, of the 
 germinal vesicle in the fowl's ovarian ovum (1825). This led 
 to Von Baer's discovery of the mammalian ovum (1827), which 
 first rendered possible a consistent view of mammalian gene- 
 ration. 
 
 The study of invertebrate embryology has, 'on the other 
 hand, during the last few years produced the most striking 
 results. 
 
 In the following pages we propose to follow in the path 
 thus marked out by the history of the subject. We begin 
 with the chick as being the animal which has been most 
 studied, and the study of which is easiest, and most fruitful 
 for the beginner. The first part accordingly will be devoted 
 to a description of the changes undergone by an incubated 
 hen's egg, especially during the early days of incubation. We 
 shall endeavour to explain, with such details as are necessary, 
 
INTRODUCTION. 7 
 
 the manner in which the embryo is formed, and the way in 
 which the rudiments of the most important organs of the 
 chick arise. We shall follow a chronological order, tracing out 
 the changes day by day (or with even shorter periods), during 
 the first few days. We are convinced that this method 
 (adopted by Von Baer) is on the whole the one which most 
 commends itself to the learner. It has of course its disad- 
 vantages ; and in several instances we have found it desirable 
 when describing, at its appropriate date, the most striking 
 phase in the development of an organ, at once to follow up 
 the subsequent history, instead of giving it piecemeal after- 
 wards. But the general advantages of the chronological 
 method, especially when the reading of such a book as this 
 is rendered really useful by an Accompanying actual exami- 
 nation of incubated eggs, are so great that they far outweigh 
 the evil of any such slight irregularities. After tracing out 
 the history of the several organs, no farther than is necessary 
 to give a clear idea of the general course of events in each 
 case, we propose to treat the changes and incidents of the 
 latter days of incubation with great brevity, not attempting 
 any special account of avian development, except in the case 
 of the skull. And even this will be treated summarily. The 
 First Part will therefore really be an introduction to the 
 general facts of vertebrate embryology, the chick being taken 
 as an example. 
 
 In the Second Part we purpose to consider the embryonic 
 histories of other vertebrates, in so far as these differ from that 
 of the bird ; and then to treat of the development of special 
 organs in a more complete manner. 
 
 The Third Part will be devoted to an exposition of the 
 main facts of invertebrate embryology, and to the discussion 
 of general morphological considerations. 
 
8 ON EMBRYOLOGY. 
 
 The reader will scarcely fail to notice that the First Part 
 especially is entirely confined to a simple description of 
 observed facts, no attempt whatever being made to interpret 
 their meanings. We have purposely pursued this course, 
 because any interpretation of the facts of the bird's develop- 
 ment is impossible, or at least illusory, till the history of other 
 animals, vertebrate and invertebrate, has been studied. 
 When all the facts are before him the reader will be in a 
 position to judge of the interpretations offered. 
 
PART I. 
 THE HISTORY OF THE CHICK. 
 
CHAPTER I. 
 
 THE STRUCTURE OF THE HEN'S EGG, AND THE CHANGES 
 WHICH TAKE PLACE UP TO THE BEGINNING OF INCUBATION. 
 
 1. IN a hen's egg quite newly laid we meet with the 
 following structures. Most external is the shell (Fig. 1, .), 
 composed of an organic basis, impregnated with calcic salts. 
 It is sufficiently porous to allow of the interchange of gases 
 between its interior and the external air, and thus the 
 chemical processes of respiration, feeble at first, but gradually 
 increasing in intensity, are carried on during the whole 
 period of incubation. 
 
 According to Nathusius, Zeitsch. f. Wiss. Zool. Vol. xvm. p. 225 270, 
 XIX. 322348, XX. 106 120, xxi. 330 355, the egg-shell of birds consists 
 of an outer thinner and an inner thicker layer. The outer layer varies con- 
 siderably in its consistency in different species. It is soft and pliant in the hen, 
 but in many other birds, as for instance the ostrich, is hard and friable. It is 
 frequently striated both vertically and transversely. Pigment when present is 
 confined to this layer. The inner layer is thicker ; and its internal surface is 
 marked with rounded processes more or less separated from one another, whose 
 blunt extremities are sunk into the shell-membrane. The presence of these pro- 
 cesses must be considered as universal amongst birds. Vertical sections shew 
 that this layer is composed of alternating horizontal laminae of transparent and 
 opaque material, the opaque laminae being composed of exceedingly minute par- 
 ticles of an organic nature imbedded in a matrix impregnated with calcic salts. 
 
 Both layers of the shell are pierced by vertical canals, which are simple in 
 Carinate but ramified in Katite birds. These canals open freely on the exterior 
 surface and also on the interior surface in the pits between the blunt processes 
 of the inner layer. It is probable that the outer openings of these canals 
 become closed by the presence of moisture, so that when the shell is wet neither 
 air nor water can pass through it. If the shell is dry, air will penetrate easily ; 
 and if the upper layer with the free ends of the tubes be rubbed off, both water 
 and air will pass through it without difficulty. In eggs with coloured shells 
 the colouring matter frequently passes into the canals. 
 
12 THE HEN'S EGG. [CHAP. 
 
 2. Lining the shell, is the shell-membrane, which is 
 double, being made up of two layers; an outer thicker 
 (Fig. 1, s. m.), and an inner thinner one (i. s. m.). Both of 
 these layers consist of several laminae of felted fibres of l 
 various sizes, intermediate in nature between connective 
 and elastic fibres. 
 
 FIG. i. 
 
 ch.l. 
 
 DIAGRAMMATIC SECTION OF AN UNTNCUBATED FOWL'S EGG 
 (modified from Allen Thomson). 
 
 bl. blastoderm, w. y. white yolk. This consists of a central flask-shaped 
 mass and a number of layers concentrically arranged around this. 
 y. y. yellow yolk. v. t. vitelline membrane, x. layer of more fluid 
 albumen immediately surrounding the yolk. w. albumen consisting of 
 alternate denser and more fluid layers, ch. I. chalaza. a. ch. air-chamber 
 at the broad end of the egg. This chamber is merely a space left between 
 the two layers of the shell-membrane, i. s. m. internal layer of shell- 
 membrane, s. m. external layer of shell-membrane, s. shell. 
 
 Over the greater part of the egg the two layers of the 
 shell-membrane remain permanently in close apposition 
 to each other; but at the broad end they tend to separate, 
 and thus to develope between them a space into which air 
 finds its way. This air-chamber, as it is called, is not to be 
 found in perfectly fresh eggs, but makes its appearance in 
 
I.] THE WHITE OF THE EGG. 13 
 
 eggs which have been kept for some time, whether incubated 
 or not, and gradually increases in size, as the white of the 
 egg shrinks in bulk by evaporation. 
 
 3. Immediately beneath the shell-membrane is the white 
 of the egg or albumen (Fig. 1, w.\ which is, chemically 
 speaking, a mixture of various forms of proteid material, 
 with fatty, extractive, and saline bodies. 
 
 Its average composition may be taken as 
 1 2*0 p. c. proteid matter, 
 I "5 p. c. fat and extractives, 
 
 5 p. c. saline matter, chiefly sodic and potassic chlorides, with phos- 
 phates and sulphates, 
 86'o p. c. water. 
 
 The white of the egg when boiled shews in section 
 alternate concentric layers of a transparent and of a finely 
 ^granular opaque material. In the natural condition, the layers 
 corresponding to these opaque layers are composed of more 
 fluid albumen, while those corresponding to the transparent 
 layers are less fluid, and consist of networks of fibres, con- 
 taining fluid in their meshes. The outer part of the white, 
 especially in eggs which are not perfectly fresh, is more fluid 
 than that nearer the yolk. The innermost layer, however, 
 immediately surrounding the yolk (Fig. 1, a?.), is of the more 
 fluid finely granular kind. 
 
 In eggs which have been hardened a spiral arrangement 
 of the white may be observed, and it is possible to tear off 
 laminae in a spiral direction from left to right, from the 
 broad to the narrow end of the egg. 
 
 Two twisted cords called the Chalazce (Fig. 1, ch. L), com- 
 posed of coiled membranous layers of the less fluid albumen, 
 run from the two extremities of the egg to the opposite 
 portions of the yolk. Their inner extremities expand and 
 merge into the layer of denser albumen surrounding the 
 fluid layer next the yolk. Their outer extremities are free, 
 and do not quite reach the outer layer of the white. Thus 
 they cannot serve to suspend the yolk, although they may 
 help to keep it in position, by acting as elastic pads. The 
 interior of each chalaza presents the appearance of a suc- 
 cession of opaque white knots ; hence the name chalazse, 
 grandines (hailstones). 
 
 4. The yolk is enclosed in the vitelline membrane 
 (Fig. 1, v. .), a transparent somewhat elastic membrane easily 
 
14 THE HEN'S EGG. [CHAP. 
 
 thrown into creases and wrinkles. It might almost be called 
 structureless, but under a high power a fine fibrillation is 
 visible, and a transverse section has a dotted or punctated 
 appearance ; it is probably therefore composed of fibres. 
 Its affinities are with elastic rather than connective tissue. 
 
 The vitelline membrane of most vertebrates is perforated by fine pores. 
 These are largest in osseous fishes and much finer in mammals; they have not 
 been found in the vitelline membrane of birds. 
 
 5. The whole space within the vitelline membrane is 
 occupied by the yolk. To the naked eye this appears toler- 
 ably uniform throughout, except at one particular point of 
 its surface, at which may be seen, lying immediately under 
 the vitelline membrane, a small white disc, about 4 mm. in 
 diameter. This is the blastoderm, or cicatricula. 
 
 A tolerably typical cicatricula in a fecundated egg will 
 shew an outer white rim of some little breadth, and within 
 that a circular transparent area, in the centre of which, 
 again, there is an opacity, varying in appearance, sometimes 
 uniform, and sometimes dotted. 
 
 The disc is always found to be uppermost whatever be 
 the position of the egg, provided there is no restraint to 
 the rotation of the yolk. The explanation of this is to be 
 sought for in the lighter specific gravity of that portion of 
 the yolk which is in the neighbourhood of the disc, and the 
 phenomenon is not in any way due to the action of the 
 chalazae. 
 
 A section of the yolk of a hard-boiled egg will shew that 
 it is not perfectly uniform throughout, but that there is a 
 portion of it having the form of a flask, with a funnel- 
 shaped neck, which, when the egg is boiled, does not become 
 so solid as the rest of the yolk, but remains more or less fluid. 
 
 The expanded neck of this flask-shaped space is situated 
 immediately underneath the disc, while its bulbous enlarge- 
 ment is about the middle of the yolk. We shall return to 
 it directly. 
 
 6. The great mass of the yolk is composed of what is 
 known as the yellow yolk (Fig. 1, y. y.). This consists of 
 spheres (Fig. 2, A.} of from 25/4 to 100/x 1 in diameter, never 
 containing a nucleus, but filled with numerous minute highly 
 refractive granules ; these spheres are very delicate and easily 
 
 = 'ooi mm. 
 
I.] THE WHITE YOLK. 15 
 
 destroyed by crushing. When boiled or otherwise hardened 
 in situ, they assume a polyhedral form, from mutual pressure. 
 The granules they contain seem to be of an albuminous 
 nature, as they are insoluble in ether or alcohol. 
 
 Chemically speaking the yolk is characterized by the presence in large 
 quantities of a proteid matter, having many affinities with globulin, and called 
 vitellin. This exists in peculiar association with the remarkable body Lecithin. 
 (Compare Hoppe-Seyler, Hdb. Phys. Chem. Anal.) Other fatty bodies, 
 colouring matters, extractives (and, according to Dareste, starch in small quan- 
 tities), &c. are also present. Miescher (Hoppe-Seyler, Chem. Untersuch. p. 502) 
 states that a considerable quantity of nuclein may be obtained from the yolk, 
 probably from the spherules of the white yolk. 
 
 FIG. 2. 
 
 A. Yellow yolk-sphere filled with fine granules. The outline of the sphere 
 has been rendered too bold. 
 
 B. White yolk-spheres and spherules of various sizes and presenting different 
 appearances. (It is very difficult in a woodcut to give a satisfactory repre- 
 sentation of these peculiar structures.) 
 
 7. The yellow yolk thus forming the great mass of the 
 entire yolk is clothed externally by a thin layer of a different 
 material, known as the white yolk, which at the edge of 
 the blastoderm passes underneath the disc, and becoming 
 thicker at this spot forms, as it were, a bed on which the 
 blastoderm rests. Immediately under the middle of the 
 blastoderm this bed of white yolk is connected, by a narrow 
 neck, with a central mass of similar material, lying in the 
 middle of the yolk (Fig. 1, w. y.}. When boiled, or otherwise 
 hardened, the white yolk does not become so solid as the 
 yellow yolk; hence the appearances to be seen in sections 
 of the hardened yolk. The upper expanded extremity of 
 this neck of white yolk is generally known as the "nucleus of 
 Pander." 
 
 Concentric to the outer enveloping layer of white yolk 
 there are within the yolk other inner layers of the same 
 substance, which cause sections of the hardened yolk to 
 
16 THE HEN'S EGG. [CHAP. 
 
 appear to be composed of alternate concentric thicker laminae 
 of darker (yellow) yolk, and thinner laminae of lighter (white) 
 yolk (Fig. 1, w, y.). 
 
 8. The microscopical characters of the white yolk are 
 very different from those of the yellow yolk. It is composed 
 of spheres (Fig. 2, S.) for the most part smaller than those of 
 the yellow yolk (4/x 75/z,), with a highly refractive nucleus- 
 like body often as small as l/^ in the interior of each; and 
 also of larger spheres, each of which contains a number of 
 spherules, similar to the smaller spheres ; these latter appear- 
 ing to have passed into the larger spheres, by a process of 
 inclusion. 
 
 There has been a considerable amount of controversy as to whether these 
 elements possess a membrane ; there is little doubt however that there is no 
 membrane present. 
 
 It has also been disputed as to whether they should be considered as true 
 cells or not. If by definition a cell must contain a nucleus, they can hardly 
 be considered as such, since the characters of the highly refractive bodies con- 
 tained in them have nothing in common with nuclei. We shall give later on 
 reasons for thinking that they may however, as a result of incubation, become 
 veritable cells. 
 
 Another feature of the white yolk, according to His, is 
 that in the region of the blastoderm it contains numerous 
 large vacuoles rilled with fluid ; they are sufficiently large to 
 be seen with the naked eye, but do not seem to be present in 
 the ripe ovarian ovum. 
 
 9. It is now necessary to return to the blastoderm. In 
 this, as we have already said, the naked eye can distinguish 
 an opaque white rim surrounding a more transparent central 
 area, in the middle of which again is a white spot of variable 
 appearance. In an unfecundated cicatricula the white disc 
 is simply marked with a number of irregular clear spaces, 
 there being no proper division into a transparent centre and 
 an opaque rim. 
 
 The opaque rim is the commencement of what we shall 
 henceforward speak of as the area opaca ; the central trans- 
 parent portion is in the same way the beginning of the area 
 pellucida. At this stage the distinction between these two 
 areas depends entirely on the disposition of the white yolk 
 beneath them, for the blastoderm when lifted up from the 
 white yolk on which it rests appears uniform throughout. 
 In the part corresponding to the area opaca the blastoderm 
 rests immediately on the white yolk, which here forms a 
 
I.] THE BLASTODERM. 17 
 
 somewhat raised ring, often spoken of as the germinal wall ; 
 underneath the area pellucida is a shallow space containing 
 a nearly clear fluid, to the presence of which the central 
 transparency seems to be due. The white spot in the middle 
 of the area pellucida appears to be the nucleus of Pander 
 shining through. 
 
 Vertical sections of the blastoderm shew that it is formed 
 of two layers. The upper of these two layers is com- 
 posed, see Fig. 3, ep, of a single layer of cells, with their 
 long axes arranged vertically, adhering together so as to 
 form a distinct membrane, the edge of which rests upon 
 the white yolk. After staining with silver nitrate, this 
 membrane viewed from above shews a mosaic of uniform 
 polygonal cells. 
 
 Each cell is composed of granular protoplasm filled with 
 highly refractive globules; in most of the cells an oval 
 nucleus may be distinguished, and is most probably present 
 in all. They are of a uniform size (about 9 /JL) over the 
 opaque and the pellucid areas. 
 
 The under layer (Fig. 8, Z), is composed of cells which 
 vary considerably in diameter ; but even the smaller cells of 
 this layer are larger than the cells of the upper layer. 
 
 They are spherical, and so filled with granules and highly 
 refractive globules, that a nucleus can rarely be seen in them: 
 in the larger cells these globules contain a highly refractive 
 body very similar to that present in the white yolk spheres, 
 from the smaller kinds of which indeed they are scarcely 
 distinguishable. 
 
 The cells of this layer do not form a distinct membrane 
 like the cells of the upper layer, but lie as a somewhat 
 irregular network of cells between the upper layer and the 
 bed of white yolk on which the blastoderm rests. The 
 lowest are generally the largest; in addition we find a 
 few still larger cells generally separated by a small interval 
 from the remainder of the cells of the lower layer, and 
 resting directly upon the white yolk (Fig. 3, 6). These are 
 frequently spoken of as formative cells; they are however 
 similar in character and indeed connected by gradations 
 with the larger cells of the lower layer. Their mode of 
 formation during segmentation will be subsequently de- 
 scribed. ' 
 
 E. 2 
 
18 
 
 THE HEN'6 EGG. 
 
 [CHAP. 
 
 FIG. 3. 
 
 SECTION OF A BLASTODEBM OF A FOWL'S EGG AT THE 
 
 <foMMENCEMENT OF INCUBATION. 
 
 The thin but complete upper layer ep composed of 
 columnar cells rests on the incomplete lower layer /, 
 composed of larger and more granular bodies. The lower 
 layer is thicker in some places than in others, and is 
 especially thick at the periphery. The line below the 
 under layer marks the upper surface of the white yolk* 
 The larger so-called formative cells are seen at 6, lying 
 on the white yolk. The figure does not take in quite 
 the whole breadth of the blastoderm ; but the reader must 
 understand that both to the right hand and to the left ep 
 is continued farther than /, so that at the extreme edge 
 it rests directly on the white }olk. 
 
 Over nearly the whole of the blastoderm 
 the upper layer rests on the under layer. 
 At the circumference however the upper 
 layer stretches for a short distance beyond 
 the under layer, and here consequently rests 
 directly on the white yolk, and forms that 
 part of the blastoderm known as the area 
 opaca* 
 
 10. To recapitulate : In the normal 
 Unincubated hen's egg we recognize the 
 blastoderm, consisting of a complete upper 
 layer of smaller nucleated granular cells and 
 a more or less incomplete under layer of 
 larger cells, filled with larger granules ; in 
 these lower cells nuclei are rarely visible. 
 The thin flat disc so formed rests, at the 
 uppermost part of the entire yolk, on a bed 
 of white yolk so disposed as to give rise to 
 the appearance in the blastodermic disc it- 
 self of an area opaca and an area pellucida. 
 The great mass of the entire yolk consists of 
 the so-called yellow yolk composed of gra- 
 nular spheres. The white yolk is composed 
 of smaller spheres of peculiar structure, and 
 exists, in small part, as a thin coating around, 
 and as thin concentric laminae in the sub- 
 ** * stance of the yellow yolk, but chiefly in the 
 
 form of a flask-shaped mass in the interior 
 of the yolk, the upper somewhat expanded top of the neck 
 
I.] THE OVARIAN OVUM. 19 
 
 of which forms the bed on which the blastoderm rests. The 
 whole yolk is invested with the vitelline membrane, this 
 again with the white ; and the whole is covered with two 
 shell-membranes and a shell. 
 
 11. Such an egg has however undergone most important 
 changes while still within the body of the hen ; and in 
 order to understand the nature of the structures which have 
 just been described, it will be necessary to trace briefly the 
 history of the egg from the stage when it exists as a so-called 
 ripe ovarian ovum in the ovary of a hen up to the time when 
 it is laid. 
 
 If one of the largest capsules of the ovary of a hen which 
 is laying regularly be opened, it will be found to contain a 
 nearly spherical (or more correctly, ellipsoidal with but 
 slightly unequal axes) yellow body enclosed in a delicate 
 membrane. This is the ovarian ovum or egg. Examined 
 with care the ovum, which is tolerably uniform in appearance, 
 will be found to be marked at one spot (generally lacing the 
 stalk of the capsule and forming the pole of the shorter axis 
 of the ovum) by a small disc differing in appearance from 
 the rest of the ovum. This disc is known as the germinal 
 disc or discus proligerus. It consists of a lenticular mass of 
 
 w.y. 
 
 SECTION THROUGH THE GERMINAL Disc OF THE RIPE OVARIAN OVUM OP A 
 
 FOWL WHILE YET ENCLOSED IN ITS CAPSULE. 
 
 a. Connective-tissue capsule of the ovum. b. epithelium of the capsule, at the 
 surface of which nearest the ovum lies the vitelline membrane, c. granular 
 material of the germinal disc, which becomes converted into the blastoderm. 
 (This is not very well represented in the woodcut. In sections which 
 have been hardened in chromic acid it consists of fine granules.) w. y, 
 white yolk, which passes insensibly into the fine granular material of the 
 disc, x, germinal vesicle enclosed in a distinct membrane, but shrivelled 
 up by the action of the chromic acid. The material enclosed in the 
 membrane of the vesicle is in the hardened specimens finely granular, 
 y, space originally completely filled up by the germinal vesicle, before the 
 latter was shrivelled up by the action of the chromic acid. 
 
 22 
 
20 THE HEN'S EGG. [CHAP. 
 
 protoplasm (Fig. 4, c), imbedded in which is a highly refrac- 
 tive globular or ellipsoidal body (Fig. 4, x), about 310/u, in 
 diameter, called the germinal vesicle, in the interior of which 
 again is a small body, the germinal spot. 
 
 The rest of the ovum is known as the yolk. This consists 
 of two elements, the white yolk- and the yellow yolk-spheres, 
 which are distributed respectively very much in the same 
 way as in the laid egg, the yellow yolk forming the mass of 
 the ovum, and the white yolk being gathered underneath and 
 around the disc (Fig. 4, w. y\ and also forming a flask- 
 shaped mass in the interior of the ovum. The delicate 
 membrane surrounding the whole is the vitelline membrane. 
 
 Oellacher's (Untersuchung uber die Furchung und BlatterUldung in Huh- 
 nereie. Studien aus dem Institute fur experimental pathologic in Wien aus 
 dem Jahre 1869, pt. l) account of the ovarian ovum differs considerably 
 from that given above. He finds in the neighbourhood of the blastoderm 
 a finely granular material, within which lies a body appearing circular when 
 viewed from above, but having in section a somewhat quadrilateral shape; 
 its side-walls, however, are curved, with their convexity turned inwards. At 
 the bottom of it lies an oval cavity with doubly contoured walls, and at its 
 upper surface placed somewhat excentrically a semicircular space filled with 
 clear material. 
 
 Oellacher believes that the quadrilateral body which he thus describes is 
 the germinal vesicle which has commenced to undergo a retrogressive meta- 
 morphosis. For the further stages in the metamorphosis, and for further par- 
 ticulars, vide Section 13. The circular hole beneath the vesicle is probably 
 merely filled with fluid and is due to the contractions of the germ. 
 
 12. When the ovarian ovum is ripe and about to be dis- 
 charged from the ovary, its capsule is clasped by the dilated 
 termination of the oviduct. The capsule then bursts, and 
 ,the ovum escapes into the oviduct, its longer axis corre- 
 sponding with the long axis of the oviduct, the germinal disc 
 therefore being to one side. At the time of the bursting of 
 the capsule the germinal vesicle disappears. 
 
 In describing the changes which take place in the 
 oviduct, it will be convenient, following the order previously 
 adopted, to treat first of all of the formation of the accessory 
 parts of the egg. These are secreted by the glandular walls 
 of the oviduct. This organ therefore requires some descrip- 
 tion. It may be said to consist of four parts ; 1st. The 
 dilated proximal extremity. 2nd. A long tubular portion, 
 opening by a narrow neck or isthmus into the 3rd portion, 
 which is much dilated, and has been called the uterus; the 
 ith part is somewhat narrow, and leads from the uterus 
 
I.] DESCENT OF THE OVUM. 21 
 
 into the cloaca. The whole of the mucous membrane lining 
 the oviduct is largely ciliated. 
 
 The accessory parts of the egg are entirely formed in the 
 2nd and 3rd portions. The layer of albumen which imme- 
 diately surrounds the yolk is first deposited; the chalazae are 
 next formed. Their spiral character arid the less distinctly 
 marked spiral arrangement of the whole albumen is brought 
 about by the motion of the egg along the spiral ridges into 
 which the interior of the second or tubular portion of the 
 oviduct is thrown. The spirals of the two chalazae are in 
 different directions. This is probably produced by their 
 peripheral ends remaining fixed while the yolk to which 
 their central ends are attached is caused to rotate by the 
 contractions of the oviduct. During the formation of the 
 chalazae the rest of the albumen is also deposited; and 
 finally the shell-membrane is formed in the narrow neck 
 of the 2nd portion, by the fibrillation of the most external 
 layer of albumen. The egg passes through the 2nd portion 
 in little more than 3 hours. In the 3rd portion the shell is 
 formed. The mucous membrane of this part is raised into 
 numerous flattened folds, like la,rge villi, containing follicu- 
 lar glands. From these a thick white fluid is poured out, 
 which ' soon forms a kind of covering to the egg, in which 
 the inorganic particles are deposited. In this portion of the 
 oviduct the egg remains from 12 to 18 hours, during which 
 time the shell acquires its normal consistency. At the time 
 of laying it is expelled from the uterus by violent muscular 
 contractions, and passes with its narrow end downwards 
 along the remainder of the oviduct, to reach the exterior. 
 
 13. We have now to trace out the changes which take 
 place in the germinal disc, during the passage of the egg 
 down the oviduct. 
 
 By the time when the egg becomes clasped by the expanded extremity of 
 the oviduct the germinal vesicle has, according to Oellacher (loc cit. and also 
 Archiv. fur Micr. Anat. Vol. vin. 1872. p. 18), undergone still further 
 retrogressive changes. It has now become very much flattened and closely 
 applied to the vitelline membrane. Both this and former stages, if we may 
 judge from the analogy of osseous fishes, are preparatory to the whole germinal 
 vesicle being bodily ejected from the germinal disc. For further particulars 
 vide Oellacher, Archiv. fur Micr. Anat. Vol. vill. pp. i 26. 
 
 Impregnation occurs in the upper portion of the oviduct ; 
 the spermatozoa being found actively moving in 'a fluid which 
 is there contained. 
 
22 THE HEN'S EGG. [CHAP. 
 
 It is not certain whether impregnation takes place previous to the deposition 
 of the albumen, or whether the spermatozoa bore their way through the albumen. 
 The former would appear to be the more probable view, though the fact that 
 Oellacher has found spermatozoa in the albumen, speaks in favour of their being 
 involved in the depositing albumen, and so being brought in contact with tiie 
 blastoderm. 
 
 According to Coste, Histoire du developpement des corps organizes, the access 
 of the cock to the hen once in seven days is sufficient. 
 
 We have no positive evidence that the spermatozoa make 
 their way through the vitelline membrane arid so gain access 
 to the germinal disc; but, as will be seen in a later part of 
 this work, analogy renders such an event probable. 
 
 14. At about the time when the shell is being formed 
 round the egg, the germinal disc undergoes a remarkable 
 change, known as segmentation. We shall have occasion to 
 treat more fully of the nature of segmentation when we 
 come to consider the amphibian ovum in which the various 
 steps of the process may be more easily and satisfactorily 
 traced. Meanwhile, inasmuch as the segmentation of the 
 
 SURFACE VIEWS OP THE EARLY STAGES OF THE SEGMENTATION IN A 
 FOWL'S EGG. (After Coste.) 
 
 A represents the earliest stage. The first furrow (b) has begun to make its 
 appearance in the centre of the germinal disc, whose periphery is marked by 
 the line a. In , the first furrow is completed right across the disc, and a 
 second similar furrow at nearly right angles to the first has appeared. The disc 
 thus becomes divided somewhat irregularly into quadrants by four (half) furrows. 
 In a later stage (C) the meridian furrows b have increased in number, from 
 four, as in Z?, to nine, and cross furrows have also made their appearance. The 
 disc is thus cut up into small central (c) and larger peripheral (d) segments. 
 Several new cross furrows are seen just beginning, as ex. gr. close to the end of 
 the line of reference d. 
 
I.] 
 
 SEGMENTATION. 
 
 23 
 
 germinal disc of a hen's egg differs materially from the 
 segmentation of the entire ovum of an amphibian, the former 
 may briefly be described here. 
 
 Viewed from above, a furrow is seen to make its appear- 
 ance, running across the germinal disc and dividing it into 
 two halves (Fig. 5, A). This primary furrow is succeeded 
 by a second at right angles to itself. The surface thus 
 becomes divided into four segments or quadrants (Fig. 5, B). 
 Each of these is again bisected by radiating furrows, and 
 thus the number of segments is increased from four to 
 eight (it may be seven or nine). The central portion of 
 each segment is then, by a cross furrow, cut off from the 
 peripheral portion, giving rise to the appearance of a number 
 of central smaller segments, surrounded by more external 
 elongated segments (Fig. 5, C). 
 
 Division of the segments now proceeds rapidly by means 
 of furrows running apparently in all directions. And it is 
 
 FIG. 6. 
 
 SURFACE VIEW OF THE GERMINAL Disc OF A HEN'S EGG DURING THE LATER 
 STAGES OF SEGMENTATION., (Chromic Acid Preparation.) 
 
 At c in the centre of the disc the segmentation masses are very small and 
 numerous. At 6. nearer the edge, they are larger and fewer; while those at 
 the extreme margin a are largest and fewest of all. It will be noticed that the 
 radiating furrows marking off the segments a do riot as yet reach to the extreme 
 margin e of the disc. 
 
 The drawing is completed in one quadrant only; it will of course be under- 
 stood that the whole circle ought to be filled up in a precisely similar manner. 
 
THE HEN'S EGG. 
 
 [CHAP. 
 
 important to note that the central segments divide more 
 rapidly than the peripheral, and consequently become at once 
 smaller and more numerous (Fig. 6). 
 
 Meanwhile sections of the hardened blastoderm teach us 
 that segmentation is not confined to the surface, but extends 
 through the mass of the blastoderm; they shew us moreover 
 that division takes place by means of not only vertical, but 
 also horizontal furrows, i.e. furrows parallel to the surface of 
 the disc (Fig. 7). 
 
 FIG. 7. 
 
 SECTION OF THE GERMINAL Disc OF A FOWL DURING THE LATER STAGES 
 
 OF SEGMENTATION. 
 
 The section, which represents rather more than half the breadth of the 
 blastoderm (the middle line being shewn at c), shews that the upper and central 
 parts of the disc segment faster than those below and towards the periphery. 
 At the periphery the segments are still very large. One of the larger segments 
 is shewn at a. In the majority of segments a nucleus can be seen ; and it seems 
 probable that a nucleus is present in them all. Most of the segments are filled 
 with highly refracting spherules^ but these are more numerous in some cells 
 (especially the larger cells near the yolk) than in others. In the central part of 
 the blastoderm the upper cells have commenced to form a distinct layer. No 
 segmentation-cavity is present. 
 
 a. large peripheral cell. b. larger cells of the lower parts of the blastoderm. 
 c. middle line of blastoderm, e. edge of the blastoderm adjoining the white 
 yolk. w. white yolk. 
 
 In this way, by repeated division, or segmentation, the 
 original germinal disc is cut up into a large number of small 
 rounded masses of protoplasm, which are smallest in the 
 centre, and increase in size towards the periphery. The 
 segments lying uppermost are moreover smaller than those 
 beneath, and thus the establishment of the two layers of 
 the blastoderm is foreshadowed. 
 
I.] SEGMENTATION. 25 
 
 According to Oellacher, Studien aus dem Ins. f. Exper. Pathol. Vien. 1869, 
 p. I, sections taken through the centre of the germinal d ; sc at the beginning 
 of segmentation shew a somewhat uneven vertical furrow, ending below in a 
 small triangular space, where it joins a nearly horizontal furrow which meets 
 the surface of the egg at some little distance on either side of the vertical 
 furrow. It seems certain that these first-formed furrows do not include the 
 \vhole of the germinal disc, whose limits at this st:<ge are however uncertain. 
 In the later stages of segmentation not only do the first-formed segments 
 become further divided, but segmentation also extends into the remainder of 
 the germinal disc. Goette, Arckiv. Micr. Anat. x. 145, indeed maintains that 
 segmentation (at a later period) even involves material which is undoubtedly 
 white yolk. He describes nuclei as making their appearance in the upper 
 surface of the bed of white yolk, and the substance round them as rising up in 
 the form of papilhe, which are subsequently constricted off and set free as 
 supplementary segmentation masses. It is these, according to him, which give 
 rise to the formative cells spoken of in the next paragraph. He states that they 
 continue to be formed long after the commencement of incubation. We shall 
 return to this subject, when we come to discuss more fully the nature of the 
 process of segmentation, in describing the ova of other classes of vertebrates. 
 
 Between the segmented germinal disc, which we may 
 now call the blastoderm, and the bed of white yolk on which 
 it rests, a space containing fluid makes its appearance. This, 
 gradually increasing in all dimensions, may be called the 
 segmentation-cavity. 
 
 15. As development proceeds, segmentation reaches its 
 limits in the centre, but continues at the periphery, and 
 thus eventually the masses at the periphery become of the 
 same size as those in the centre. 
 
 The distinction however between an upper and a lower 
 layer becomes more and more obvious. 
 
 The masses of the upper layer arrange themselves, side 
 by side, with their long axes vertical ; their nuclei become 
 very distinct. In fact they form a membrane of columnar 
 nucleated cells. 
 
 The masses of the lower layer, remaining larger than 
 those of the upper layer, continue markedly granular and 
 round, and form rather a close irregular network than a 
 distinct membrane. In them nuclei are either wholly absent 
 or at least not readily visible. 
 
 It seems more probable that the nucleus is hidden than that it is really 
 absent. In the earliest stages of segmentation which we have examined when 
 the segments were still few in number, a very large proportion of both great 
 and small segments contained large well-formed nuclei. These nucleated 
 segments, which were found in both the superficial and deeper portions of the 
 disc, were invariably those in which the granules were for some reason or other 
 few and fine; in fact, wherever the granules were not sufficiently numerous to 
 render the body of the segment too opaque, there a nucleus could be detected. 
 
26 THE HEN'S EGG. [CHAP. i. 
 
 We were thus led to the conclusion that a nucleus really existed in all. It is 
 of course quite possible that the clearer nucleated masses eventually come to 
 the surface and leave the more granular and opaque masses to form the lower 
 layer; but it is much more likely that they do not, and that the granular con- 
 dition of the cells of the lower layer of the fully formed blastoderm is on the 
 one hand the result of their being in immediate contact with the excessively 
 granular white yolk-cells, and on the other the cause of their nuclei not being 
 seen. We have a somewhat analogous case in the invisibility of the nucleus in 
 the early stages of the amphibian blood-corpuscle. 
 
 At the time when the segmentation-spheres in the centre 
 are smaller than those at the periphery, and those above 
 are also smaller than those below, a few large spherical 
 masses begin to separate from the remainder (or to arise 
 by a continued process of segmentation from the bed of 
 white yolk), and to rest directly on the white yolk, at the 
 bottom of the shallow segmentation-cavity. They contain 
 either numerous small nucleated spherules, or fine granules ; 
 the spherules precisely resembling the smaller spheres of 
 white yolk. These loose spherical masses are the formative 
 cells already spoken of. 
 
 Thus the original germinal disc of the ovarian ovum, 
 its germinal vesicle having disappeared, becomes, by the 
 process of segmentation, converted into a blastoderm such as 
 is met with in the egg when laid, into an upper layer of 
 columnar nucleated cells, and into a lower layer of irregularly 
 disposed rounded masses which have not yet definitely ac- 
 quired the character of cells, accompanied by a few stray 
 " formative " cells lying loose in the segmentation-cavity. 
 
CHAPTER II. 
 
 A BRIEF SUMMARY OF THE WHOLE HISTORY OF INCUBATION. 
 
 1. STEP by step the simple two-layered blastoderm de- 
 scribed in the previous chapter is converted into the complex 
 organism of the chick. The details of the many changes 
 through which this end is reached will perhaps be rendered 
 more intelligible if we prefix to the special history of them 
 a brief summary of the general course of events from the 
 beginning to the end of incubation. 
 
 In the first place, it is to be borne in mind that the 
 embryo itself is formed in the area pellucida, and in the 
 area pellucida alone. The area opaca in no part enters 
 directly into the body of the chick; the structures to 
 which it gives rise are to be regarded as appendages, which 
 sooner or later disappear. 
 
 2. The blastoderm at starting consists of two layers. 
 Very soon a third layer makes its appearance between the 
 other two. These three layers, the establishment of which 
 is a fact of fundamental importance in the history of the 
 embryo, are called respectively the upper, middle and lower 
 layers, or epiblast, mesoblast and hypoblast. 
 
 This triple division corresponds roughly, though not exactly, to the old 
 division into serous, vascular and mucous layers. 
 
 3. The blastoderm which at first, as we have seen, lies 
 like a watch-glass over the segmentation-cavity, its margin 
 resting on the circular germinal wall of white yolk, spreads, 
 as a thin circular sheet, over the yolk, immediately under 
 the vitelline membrane. Increasing uniformly at all points 
 of its circumference, the blastodermic expansion covers more 
 and more of the yolk, and at last, reaching its opposite pole 
 
28 PRELIMINARY ACCOUNT. [CHAP. 
 
 completely envelopes it. Thus the whole yolk, instead of 
 being enclosed as formerly by the vitelline membrane alone, 
 comes to be also enclosed in a bag formed by the blastoderm. 
 
 It is not however until quite a late period that the 
 complete closing in at the opposite pole takes place, so 
 that the extension of the blastoderm must be thought of as 
 going on during nearly the whole period of incubation. 
 
 Both the area opaca and the area pellucida share in 
 this enlargement, but the area opaca increases much more 
 rapidly than the area pellucida, and plays the principal 
 part in encompassing the yolk. 
 
 4. The mesoblast, in that part of the area opaca which 
 is nearest to the area pellucida, becomes the seat of peculiar 
 changes, which result in the formation of blood-vessels. 
 Hence this part of the area opaca is called the vascular area. 
 
 5. The embryo itself may be said to be formed by a 
 folding off of the central portion of the area pellucida from 
 the rest of the blastoderm. At first the area pellucida is 
 quite flat, or, inasmuch as it forms part of the circumference 
 of the yolk, slightly but uniformly curved. Very soon, how- 
 ever, there appears at a certain spot a semilunar groove, at 
 first small, but gradually increasing in depth and extent ; this 
 groove, which is represented in section in the diagram (Fig. 
 8, A) 9 breaks the uniformity of the level of the area pellucida. 
 It may be spoken of as a tucking in of a small portion of the 
 blastoderm in the form of a crescent. When viewed from 
 above, it presents itself as a curved line (the hinder of the 
 two concentric curved lines in front of A in Fig. 11), which 
 marks the hind margin of the groove, the depression itself 
 being hidden. In a vertical longitudinal section carried 
 through the middle line, we may recognize the following 
 parts (Fig. 8, A, or on a larger scale Fig. 9, which also 
 shews details which need not be considered now). Beginning 
 at what will become the posterior extremity of the embryo, 
 (the left-hand side of the figure in each case), and following 
 the surface of the blastoderm forwards (to the right in the 
 figures), the level is maintained for some distance, and then 
 there is a sudden descent, the blastoderm bending round and 
 pursuing a precisely opposite direction to its previous one, 
 running backwards instead of forwards, for some distance. 
 It soon however turns round again, and once more running 
 
II.] 
 
 THE HEAD-FOLD. 
 FIG. 8. 
 
 29 
 
 Fig. 8, A to N forms a series of purely diagrammatic representations in- 
 troduced to facilitate the comprehension of the manner in which the body of 
 the embryo is formed, and of the various relations of the yolk-sac, amnion 
 and allantois. 
 
 In all vt is the vitelline membrane, placed, for convenience sake, at some 
 distance from its contents, and represented as persisting in the later stages ; in 
 the actual egg it is in direct contact with the blastoderm (or yolk), and early 
 ceases to have a separate existence. In all e indicates the embryo, pp the 
 general pleuro-peritoneal space, af the folds of the amnion, a the amnion proper, 
 ae or ac the cavity holding the liquor amnii, al the allantois, a' the alimentary 
 canal, y or ys the yolk or yolk-sac. 
 
 J., which may be considered as a vertical section taken longitudinally along 
 the axis of the embryo, represents the relations of the parts of the egg at 
 the time of the first appearance of the head-fold, seen on the right-hand side 
 of the blastoderm e. The blastoderm is spreading both behind (to the left 
 hand in the figure), and in front (to right hand) of the head -fold, its limits 
 being indicated by the shading and thickening for a certain distance of the 
 margin of the yolk y. As yet there is no fold on the left side of e correspond- 
 ing to the head-fold on the right, while therefore the front limit of the embryo, 
 as distinguished from the blastoderm, is marked out by the head-fold, there 
 is at present no tail-fold, and therefore no hind limit. 
 
 B is a vertical transverse section of the same period drawn for convenience 
 sake on a larger scale (it should have been made flatter and less curved). It 
 shews that the blastoderm (vertically shaded) is extending laterally as well as 
 fore and aft, in fact in all directions; but there are no lateral folds, and there- 
 
30 
 
 PRELIMINARY ACCOUNT. 
 
 [CHAP. 
 
 fore no lateral limits to the body of the embryo as distinguished from the 
 blastoderm. 
 
 Incidentally it shews the formation of the medullary groove by the rising 
 up of the laminae dorsales. Beneath the section of the groove is seen the 
 rudiment of the notochord. On either side a line indicates the cleavage of 
 the mesoblast just commencing. This cleavage, it will be seen, does not exist 
 in the more central parts of the embryo. 
 
 In C, which represents a vertical longitudinal section of later date, both 
 head-fold (on the right) and tail-fold (on the left) have advanced considerably. 
 The alimentary canal is therefore closed in both, in front and behind, but is in 
 middle still widely open to the yolk y below. Though the axial parts of the 
 embryo have become thickened by growth, the body-walls are still thin; in 
 them however is seen the cleavage of the mesoblast, and the divergence of 
 the somatopleure and splanchnopleure. The splanchnopleure both at the head 
 and at the tail is folded in to a greater extent than the somatopleure, and 
 
 PP-1-,::. 
 
 E 
 
 It 
 
 ^ 
 
II.] 
 
 THE BODY-FOLDS. 31 
 
 forms the still wide splanchnic stalk. At the end of the stalk, which is as yet 
 short, it bends outwards again and spreads over the top of the yolk. The 
 somatopleure folded in less than the splanchnopleure to form the wider somatic 
 stalk, sooner bends round and runs outwards again. At a little distance from 
 both the head and the tail it is raised up into a fold, af, af, that in front of the 
 head being the highest. These are the amniotic folds. Descending from either 
 fold, it speedily joins the splanchnopleure again, and the two, once more united 
 into an un cleft membrane, extend some way downwards over the yolk, the 
 limit or outer margin of the opaque area not being shewn. All the space 
 between the somatopleure and the splanchnopleure is shaded with dots, pp. 
 Close to the body this space may be called the pleuroperitoneal cavity; but 
 outside the body it runs up into either amuiotic told, and also extends some 
 little way over the yolk. 
 
 D represents the tail-end at about the same stage on a more enlarged 
 scale, in order to illustrate the position of the allantois al (which was for the 
 sake of simplicity omitted in (7), shewn as a bud from the splanchnopleure, 
 stretching downwards into the pleuroperitoneal cavity pp. The dotted area 
 representing as before the whole space between the splanchnopleure and the 
 somatopleure, it is evident that a way is open for the aliautois to extend from 
 its present position into the space between the two walls of the amniotic 
 fold af. 
 
 E, also a longitudinal section, represents a stage still farther advanced. 
 Both splanchnic and somatic stalks are much narrowed, especially the former, 
 the cavity of the alimentary canal being now connected with the cavity of the 
 yolk by a mere canal. The folds of the amnion are spreading over the top of 
 the embryo and nearly meet. Each fold consists of two walls or Jimbs, the 
 space between which (dotted) is as before merely a part of the space between 
 the somatopleure and splanchnopleure. Between these arched amniotic folds 
 and the body of the embryo is a space not as yet entirely closed in. 
 
 F represents on a different scale a transverse section of E taken through 
 the middle of the splanchnic stalk. The black ring in the body of the embryo 
 shews the position of the neural canal, below which is a black spot, marking 
 the notochord. On either side of the notochord the divergence of somato- 
 pleure and splanchnopleure is obvious. The splanchnopleure, more or less 
 thickened, is somewhat bent in towards the middle line, but the two sides do 
 not unite, the alimentary canal being as yet open below at this spot; after 
 converging somewhat they diverge again and run outwards over the yolk. 
 The somatopleure folded in to some extent at first to form the body-walls 
 (which are here made too thick), soon bends outwards again, and almost im- 
 mediately is raised up into the lateral folds of tbe amnion af. The conti- 
 nuity of the pleuroperitoneal cavity within the body with the interior of the 
 amniotic fold outside the body is evident; both cavities are dotted. It will 
 of course be understood that this is a purely diagrammatic representation, 
 the various cavities, &c., being exaggerated in order to shew their relations more 
 clearly. 
 
 G, which corresponds to D at a later stage, is introduced to shew the 
 manner in which the allantois, now a distinctly hollow body, whose cavity is 
 continuous with that of the alimentary canal, becomes directed towards the 
 amniotic fold. 
 
 In H a longitudinal, and / a transverse section of later date, great changes 
 have taken place. The several folds of the amnion have met and coalesced 
 above the body of the embryo. The inner limbs of the several folds have 
 united into a single membrane (a), which encloses a space (ae) round the embryo. 
 This membrane a is the amnion proper, and the cavity within it, i.e. between 
 it and the embryo, is the cavity of the amnion containing the liquor amnii. 
 
32 
 
 PRELIMINAKY ACCOUNT. 
 
 [CHAP. 
 
 It will be seen that the amnion a now forms in every direction the termina- 
 tion of the somatopleure ; the peripheral portions of the sornatopleure, the 
 united outer or descending limbs or walls of the folds af in (7, D, F, G having 
 been cut adrift, and now forming an independent continuous membrane, the 
 chorion, immediately underneath the vitelline membrane. 
 
 Jn / the splanchnopleure is seen converging to complete the closure 
 of the alimentary canal a', even at the stalk (elsewhere the canal has of course 
 long been closed in), and then spreading outwards as usual over the yolk. 
 The point at which it unites with the somatopleure, marking the extreme 
 limit of the cleavage of the mesoblast, is now much nearer the lower pole 
 of the diminished yolk. 
 
 As a result of these several changes, a great increase in the dotted space 
 has taken place. It is now possible to pass from the actual peritoneal cavity 
 within the body, on the one hand round a great portion of the circumference 
 of the yolk, and on the other hand above the amnion a, in the space between 
 it and the chorion. 
 
 Into this space the allantois is seen spreading in K at al. 
 
 M 
 
 In L the splanchnopleure has completely invested the yolk-sac, but at the 
 lower pole of the yolk is still continuous with that peripheral remnant of the 
 soraatopleure now called the chorion. In- other words, cleavage of the meso- 
 blast has been carried all round the yolk (ys) except at the very lower pole. 
 
 In M the cleavage has been carried through the pole itself; the peripheral 
 portion of the splanchnopleure forms a complete investment of the yolk quite 
 unconnected with the peripheral portion of the somatopleure, which no*w exists 
 as a continuous membrane lining the interior of the shell. The yolk sac (ys) is 
 therefore quite loose in the pleuroperitoneal cavity, being connected only with 
 the alimentary canal (a') by a solid pedicle. 
 
II.] 
 
 THE HEAD-FOLD. 
 
 33 
 
 Lastly, in N the yolk sac (?/s) is shewn being withdrawn into the cavity 
 of the body of the embryo. The allantois is as before, for the sake of 
 simplicity, omitted ; its pedicle would of course lie by the side of ys in the 
 somatic stalk marked by the usual dotted shading. 
 
 It may be repeated that the above are diagrams, the various spaces being 
 shewn distended, whereas in many of them in the actual egg the walls have 
 collapsed, and are in near juxtaposition. 
 
 c 
 
 FIG. 9. 
 
 JVC. 
 
 F.So. 
 
 DIAGRAMMATIC LONGITUDINAL SECTION THROUGH THE Axis OF AN EMBRYO. 
 
 The section is supposed to be made at a time when the head-fold has com- 
 menced but the tail-fold has not yet appeared. 
 
 F. So. fold of the somatopleure. F. Sp. fold of the splanchnopleure.* 
 
 The line of reference F. So. is placed in the lower bay, outside the embryo. 
 The line of D is placed in the upper bay inside the embryo; this will remain as 
 the alimentary canal. Both folds (F. So., F. Sp.) are parts of the head-fold, and 
 are to be thought of as continually travelling onwards (to the left) as develop- 
 ment proceeds. 
 
 pp. space between somatopleure and splanchnopleure: pleuroperitoneal cavity. 
 Am. commencing (head) fold of the amnion. 
 
 A fuller explanation is given under Fig. 16. 
 
 forward, with a gentle ascent, regains the original level. As 
 seen in section, then, the blastoderm at this spot may be said to 
 be folded up in the form of the letter 8. This fold we shall 
 always speak of as the head-fold. In it we may recognize 
 two limbs : an upper limb in which the curve is directed for- 
 wards, and its bay, opening backwards, is underneath the 
 blastoderm, i. e. as we shall see, inside the embryo (Fig. 9, D] 
 and an under limb in which the curve is directed backwards, 
 and its bay, opening forwards, is above the blastoderm, i.e. 
 outside the embryo. If an j like the above, made of some 
 elastic material, were stretched laterally, the effect would be 
 to make both limbs longer and proportionally narrower, and 
 E. 3 
 
34 PRELIMINARY ACCOUNT. [CHAP. 
 
 their bays, instead of being shallow cups, would become more 
 tubular. Such a result is in part arrived at by the growth 
 of the blastoderm; the upper limb of the 8 is continually 
 growing forward (but, unlike the stretched elastic model, in- 
 creases in all its dimensions at the -same time), and the lower 
 limb is as continually lengthening backwards ; and thus both 
 upper and lower bays become longer and longer. This we 
 shall hereafter speak of as the travelling backwards of the 
 head-fold. 
 
 The two bays do not however both become tubular. The 
 section we have been speaking of is supposed to be taken 
 vertically along a line, which will afterwards become the axis 
 of the embryo ; and the lower bay of the 3 is a section of the 
 crescentic groove mentioned above, in its middle or deepest 
 part. On either side of the middle line the groove gradually 
 becomes shallower. Hence in sections taken on either side 
 of the middle line or axis of the embryo (above or below the 
 plane of the figures), the groove would appear the less marked 
 the farther the section from the middle line, and at a certain 
 distance would disappear altogether. It must be remembered 
 that the groove is at first crescent-shaped, with its concavity 
 turned towards what will be the hind end of the embryo 
 (Fig. 11). As the whole head-fold is carried farther and farther 
 back, the horns of the crescent are more and more drawn 
 in towards the middle line, the groove becoming first semi^ 
 circular, then horse-shoe-shaped. In other words, the head- 
 fold, instead of being a simple fold running straight back- 
 wards, becomes a curved fold with a central portion in front 
 running backwards, and two side portions running in towards 
 the middle line. The effect of this is that the upper bay of 
 the 8 (that within the embryo) gets closed in at the sides 
 as well as in the front, and thus speedily becomes tubular. 
 The under bay of the 8 (that outside the embryo) remains 
 of course open at the sides as in front, and forms a sort of 
 horse-shoe-shaped ditch surrounding the front end of the 
 embryo. 
 
 We have dwelt thus at length on the formation of the 
 head-fold, because, unless its characters are fairly grasped, 
 much difficulty may be found in understanding many events 
 in the history of the chick. The reader will perhaps find the 
 matter easier to comprehend if he makes for himself a rough 
 
II.] THE EMBRYONIC SAC. 35 
 
 model, which he easily can do by spreading a cloth out flat 
 to represent the blastoderm, placing one hand underneath it, 
 to mark the axis of the embryo, and then tucking in the 
 cloth from above under the tips of his fingers. The fingers, 
 covered with the cloth and slightly projecting from the level 
 of the rest of the cloth, will represent the head, in front of 
 which will be the semicircular or horse-shoe-shaped groove 
 of the head-fold. 
 
 At its first appearance the whole 8 may be spoken of as 
 the head-fold, but later on it will be found convenient to 
 restrict the name chiefly to the lower limb of the 8. 
 
 Some time after the appearance of the head-fold, an 
 altogether similar but less conspicuous fold makes its ap- 
 pearance, at a point which will become the posterior end of 
 the embryo. This fold, which travels forwards just as the 
 head-fold travels backwards, is the tail-fold (Fig. 8, C). 
 
 In addition, between the head- and the tail-fold two lateral 
 folds appear, one on either side. These are simpler in cha- 
 racter than either head-fold or tail-fold, inasmuch as they 
 are nearly straight folds directed inwards towards the axis of 
 the body (Fig. 8, F), and not complicated by being crescentic 
 in form. Otherwise they are exactly similar. 
 
 As these several folds become more and more developed, 
 the head-fold travelling backwards, the tail-fold forwards, 
 and the lateral folds inwards, they tend to unite in the 
 middle point ; and thus give rise more and more distinctly 
 to the appearance of a small tubular sac seated upon, and 
 connected, by a continually-narrowing hollow stalk, with that 
 larger sac which is formed by the extension of the rest of the 
 blastoderm over the whole yolk. 
 
 The smaller sac we may call the "embryonic sac," the 
 larger one "the yolk-sac." As incubation proceeds the smaller 
 sac (Fig. 8), gets larger and larger at the expense of the yolk- 
 sac (the contents of the latter being gradually assimilated by 
 nutritive processes into the tissues forming the growing walls 
 of the former, not directly transferred from one cavity into 
 the other). Within a day or two of the hatching of the 
 chick, at a time when the yolk-sac is still of some consider- 
 able size, or at least has not yet dwindled away altogether, 
 and the development of the embryonic sac is nearly com- 
 plete, the yolk-sac (Fig. 8, JV) is slipped into the body of 
 
 32 
 
36 PRELIMINARY ACCOUNT. [CHAP- 
 
 the embryo, so that ultimately the embryonic sac alone re- 
 mains. 
 
 6. The embryo, then, is formed by a folding off of a 
 portion of the blastoderm from the yolk-sac. The general 
 outline of the embryo is due to the direction and shape of 
 the several folds which share in its formation ; these, while 
 preserving a nearly perfect bilateral symmetry, present marked 
 differences at the two ends of the embryo. Hence from the 
 very first there is no difficulty in distinguishing the end which 
 will be the head from that which will be the tail. 
 
 In addition to this, the tubular sac of the embryo, while 
 everywhere gradually acquiring thicker and thicker walls, 
 undergoes at various points, through local activities of growth 
 in the form of thickenings, ridges, buds or other processes, 
 many modifications of the outline conferred upon it by the 
 constituent folds. Thus bud- like processes start out from 
 the trunk to form the rudiments of the limbs, and simila,r 
 thickenings and ridges give rise to the jaws and other parts 
 of the face. By the unequal development of these outgrowths 
 the body of the chick is gradually moulded into its proper 
 outward shape. 
 
 7. Were the changes which take place of this class only, 
 the result would be a tubular sac of somewhat complicated 
 outline, but still a simple tubular sac. Such a simple sac 
 might perhaps be roughly taken to represent the body of 
 many an invertebrate animal ; but the typical structure of a 
 bird or other vertebrate animal is widely different. It may 
 very briefly be described as follows. 
 
 First there is, above, a canal running lengthways along 
 the body, in which are lodged the brain and spinal cord. 
 Below this neural tube is an axis represented by the bodies 
 of the vertebra and their continuation forwards in the 
 structures which form the base of the skull. Underneath 
 this, again, is another tube closed in above by the axis, 
 and on the sides and below by the body- walls. Enclosed 
 in this second tube, and suspended from the axis, is a 
 third tube, consisting of the alimentary canal with its 
 appendages (liver, salivary glands, lungs, &c., which are 
 fundamentally mere diverticula from one simple canal). 
 The cavity of the outer tube, which also contains the 
 heart and other parts of the vascular system, is the general 
 
II.] , THE NEURAL TUBE. 37 
 
 body cavity ; it is divided into a thoracic or pleural, and an 
 abdominal or peritoneal cavity; these two cavities are, how- 
 ever., from their mode of origin, portions of one and the same 
 tube. Thus a transverse section of a vertebrate animal 
 always shpws the same fundamental structure: above a single 
 tube, below a double tube, the latter consisting of one tube 
 enclosed within another, the inner being the alimentary 
 canal, the outer the general cavity of the body. Into such 
 a triple tube the simple tubular embryonic sac of the chick 
 is converted by a series of changes of a remarkable character. 
 
 The upper or neural tube is formed in the following way. 
 At a very early period the upper surface of the blastoderm 
 in the region which will become the embryo, is raised up 
 into two ridges or folds which run parallel to each other at a 
 short distance on either side of what will be the long axis of 
 the embryo, and thus leave between them a shallow longitu- 
 dinal groove (Fig. 8, B, also Figs. 11, 12, m.c). As these ridges, 
 which bear the name of medullary folds, increase in height 
 they arch over towards each other and eventually meet 
 and coalesce in the middle line, thus converting the groove 
 into a canal, which at the same time becomes closed at either 
 end (Fig. 8, F, /, also Fig. 13. .M.). The cavity so 
 formed is the cavity of the neural tube, and eventually 
 becomes the cerebro-spinal canal. 
 
 The lower double tube, that of the alimentary canal, and 
 of the general cavity of the body, is formed in an entirely 
 different way. It is, broadly speaking, the result of the junc- 
 tion and coalescence of the fundamental embryonic folds, the 
 head-fold, tail-fold, and lateral folds ; in a certain sense the 
 cavity of the body is the cavity of the tubular sac described 
 in the last paragraph. 
 
 But it is obvious that a tubular sac formed by the folding 
 in of a single sheet of tissue, such as we have hitherto con- 
 sidered the blastoderm to be, must be a simple tubular sac 
 possessing a single cavity only. The blastoderm however 
 does not long remain a single sheet, but speedily becomes a 
 double sheet of such a kind that, when folded in, it gives rise 
 to a double tube. 
 
 Very early the blastoderm becomes thickened in the 
 region of the embryo, the thickening being chiefly due to an 
 increase in the middle layer or mesoblast, while at the same 
 
SS PRELIMINARY ACCOUNT. [CHAP. 
 
 time it becomes split or cleft horizontally over the greater 
 part of its extent into two leaves, an upper leaf and a lower 
 leaf. In the neighbourhood of the axis of the body, beneath 
 the neural tube, this cleavage is absent (Fig. 8, B\ also Figs. 
 13 20), in fact, it begins at some little distance on either 
 side of the axis and spreads thence into the periphery in all 
 directions. It is along the thickened mesoblast that the 
 cleavage takes place, the upper part of the mesoblast uniting 
 with epiblast to form the upper leaf, and the lower part with 
 the hypoblast to form the lower leaf. 
 
 In the fundamental folds both leaves are involved, both 
 leaves are folded downwards and inwards, both leaves tend 
 to meet in the middle below; but the lower leaf is folded in 
 more rapidly, and thus diverges from the upper leaf, a space 
 being gradually developed between them (Fig. 8). In course 
 of time the several folds of the lower leaf meet and unite to 
 form an inner tube quite independently of the upper leaf> 
 whose own folds in turn meet and unite to form an outer 
 tube separated from the inner one by an intervening space. 
 The inner tube is the alimentary canal which is subsequently 
 perforated at both ends to form the mouth and anus; the 
 walls of the outer tube are the walls of the body, and the 
 space between the two tubes is the general " serous cavity/' 
 which being subsequently divided into pleural and peritoneal 
 portions, may be spoken of as the pleuroperitoneal cavity. 
 
 Hence the upper (or outer) leaf of the blastoderm, from 
 its giving rise to the body- walls, is called the somatopleure 1 ; 
 the lower (or inner) leaf, from its forming the alimentary 
 canal and its tributary viscera, the splanchnopleure 2 . 
 
 This horizontal splitting of the blastoderm into a somato- 
 pleure and a splanchnopleure, which we shall hereafter speak 
 of as the cleavage of the mesoblast, is not confined to the region 
 of the embryo, but gradually extends over the whole of the 
 yolk-sac. Hence in the later days of incubation the yolk- 
 sac comes to have two distinct coats, an inner splanclmo- 
 pleuric and an outer somatopleuric investment, separable 
 from each other all over the sac. We have seen that, owing 
 to the manner of its formation, the ' embryonic sac ' is con- 
 nected with the ' yolk-sac' by a continual narrowing hollow 
 
 1 Soma, body, pleuron, side. ? Splanchnic, viscus, pleuron y side. 
 
II.] THE AMNION. 39 
 
 stalk ; but this stalk must, like the embryonic sac itself, be 
 a double stalk, and consist of a smaller inner stalk within a 
 larger outer one, Fig. 8, E } H. The folds of the splanchnopleure, 
 as they tend to meet and unite in the middle line below, 
 give rise to a continually narrowing hollow stalk of their 
 own, a splanchnic stalk, by means of which the walls of the 
 alimentary canal are continuous with the splanchnopleuric 
 investment of the yolk-sac, and the interior of that canal is 
 continuous with the yolk inside the yolk-sac. In the same 
 way the folds of the somatopleure form a similar stalk of 
 their own, a somatic stalk, by means of which the body- 
 walls of the chick are continuous (for some time; the con- 
 tinuity, as we shall see, being eventually broken by the 
 development of the amnion) with the somatopleuric invest- 
 ment of the yolk-sac ; and the pleuroperitoneal cavity of the 
 body of the chick is continuous with the narrow space be- 
 tween the two investments of the yolk-sac. 
 
 At a comparatively early period the canal of the splanch- 
 nic stalk becomes obliterated, so that the material of the 
 yolk can no longer pass directly into the alimentary cavity, 
 but has to find its way into the body of the chick by absorp- 
 tion through the blood-vessels. The somatic stalk, on the 
 other hand, remains widely open for a much longer time ; but 
 the somatic shell of the yolk-sac never undergoes that thick- 
 ening which takes place in the somatic walls of the embryo 
 itself; on the contrary, it remains thin and insignificant. 
 When accordingly in the last days of incubation the greatly 
 diminished yolk-sac with its splanchnic investment is with- 
 drawn into the rapidly enlarging abdominal cavity of the 
 embryo, the walls of the abdomen close in and unite, without 
 any regard to the shrivelled, emptied somatopleuric invest- 
 ment of the yolk-sac, which is cast off as no longer of any 
 use. (Fig. 8. Compare the series.) 
 
 8. Very closely connected with the cleavage of the meso- 
 blast and the division into somatopleure and splanchnopleure, 
 is the formation of the amnion, all mention of which was, for 
 the sake of simplicity, purposely omitted in the description 
 just given. 
 
 The amnion takes its origin from certain folds of the 
 .somatopleure, and of the somatopleure only, in the following 
 way. 
 
40 PRELIMINARY ACCOUNT. [CHAP. 
 
 At a time when the cleavage of the mesoblast has some- 
 what advanced, there appears, a little way in front of the semi- 
 lunar head-fold, a second fold (Fig. 11, also Fig 8, (7.), running 
 more or less parallel or rather concentric with the first and 
 not unlike it in general appearance, though differing widely 
 from it in nature. In the head-fold the whole thickness of the 
 blastoderm is involved ; in it both somatopleure and splanch- 
 nopleure (where they exist, i.e. where the mesoblast is cleft,) 
 take part. This second fold, on the contrary, is limited entirely 
 to the somatopleure. Compare Figs. 8 and 9. In front of 
 the head-fold, and therefore altogether in front of the body 
 of the embryo, the somatopleure is a very thin membrane, 
 consisting only of epiblast and a very thin layer of mesoblast ; 
 and the fold we are speaking of is, in consequence, itself 
 thin and delicate. Rising up as a semilunar fold with its 
 concavity directed towards the embryo (Fig. 8, (7, a/.), 
 as it increases in height it is gradually drawn backwards 
 over the developing head of the embryo. The fold thus 
 covering the head is in due time accompanied by similar 
 folds of the somatopleure, starting at some little distance be- 
 hind the tail, and at some little distance from the sides (Fig. 8, 
 C, D, E, F). In this way the embryo becomes surrounded 
 by a series of folds of thin somatopleure, which form a 
 continuous wall all round it. All are drawn gradually over the 
 body of the embryo, and at last meet and completely coalesce 
 (Fig. 8, H, /), all traces of their junction being removed. 
 Beneath these united folds there is therefore a cavity, within 
 which the embryo lies (Fig. 8, H y ae). This cavity is the 
 cavity of the amnion. The folds which we have been 
 describing are those which form the amnion. 
 
 Each fold, of course, necessarily consists of two limbs, both 
 limbs consisting of epiblast and a very thin layer of mesoblast; 
 but in one limb the epiblast looks towards the embryo, while 
 in the other it looks away from it. The space between the two 
 limbs of the fold, as can easily be seen in Fig. 8, is really part 
 of the space between the somatopleure and splanchnopleure ; 
 it is therefore continuous with the general space, part of which 
 afterwards becomes the pleuroperitoneal cavity of the body, 
 shaded with dots in the figure and marked (p p). So that it 
 is possible to pass from the cavity between the two limbs of 
 each fold of the amnion into the cavity which surrounds 
 
II.] THE ALLANTOIS. i 41 
 
 the alimentary canal. When the several folds meet and 
 coalesce together above the embryo, they unite in such a 
 way that all their inner limbs go to form a continuous inner 
 membrane or sac, and all their outer limbs a similarly con- 
 tinuous outer membrane or sac. The inner membrane thus 
 built up forms a completely closed sac round the body of the 
 embryo, and is called the amniotic sac, or amnion proper, 
 (Fig. 8, H, /, &c. a.), and the fluid which it afterwards con- 
 tains is called the amniotic fluid, or liquor amnii. The space 
 between the inner and outer sac, being formed by the united 
 cavities of the several folds, is, from the mode of its forma- 
 tion, simply a part of the general cavity found everywhere 
 between somatopleure and splanchnopleure. The outer sac 
 over the embryo lies close under the vitelline membrane, 
 while its periphery is gradually extended over the yolk as 
 the somatopleuric investment of the yolk-sac described in the 
 preceding paragraph. 
 
 9. If the mode of origin of these two sacs (the inner 
 or true amnion, and the outer or false amnion, as Baer 
 called it) and their relations to the embryo be borne in mind, 
 the reader will have no difficulty in understanding the course 
 taken in its growth by an important organ, the allantois, of 
 which we shall hereafter have to speak more in detail. ^ 
 
 The allantois is fundamentally an appendage of the 
 alimentary canal, and may be regarded as a bud thrown 
 out by the splanchnopleure close to its junction with the 
 somatopleure at the hinder end of the embryo (Fig. 8, D, 
 al.}. From thence it grows first into the pleuroperitoneal 
 cavity of the embryo, and thence very rapidly pushes its 
 way by the development of a long stalk into the space 
 between the true and false amniotic sacs (Fig. 8, (?, K}. 
 Curving over the embryo, it comes to lie over the embryo 
 and the amnion proper, separated from the shell (and vitelline 
 membrane) by nothing more than the thin false amnion. 
 In this position it performs its functions as a respiratory 
 organ. It is evident that though now placed quite outside 
 the embryo, the space in which it lies is a continuation of 
 that peritoneal cavity in which it took its origin. 
 
 It is only necessary to add, that the false amnion either 
 coalesces with the vitelline membrane, in contact with which 
 
42 PRELIMINARY ACCOUNT. [CHAP. II. 
 
 it lies, or else replaces it, and in the later days of incuba- 
 tion is known as the chorion. 
 
 In the above account we Lave described the somatopleure as consisting 
 of mesoblast as well as epiblast even in its most peripheral portions. The inner 
 limbs of the amrriotic folds undoubtedly contain mesoblastic elements, since 
 the amnion proper contains plain muscular fibres. Some authors however 
 regard the outer limbs of the amniotic folds (giving rise to the false amnion) 
 and the somatopleuru beyond them as being composed of epiblast only. 
 
CHAPTER III. 
 
 THE CHANGES WHICH TAKE PLACE DURING THE FIRST 
 DAY OF INCUBATION. 
 
 1. DURING the descent of tlie egg along the oviduct, 
 where it is exposed to a temperature of about 40 C, the 
 blastoderm, as we have seen, continues to undergo im- 
 portant changes. When the egg is laid and becomes cold 
 these changes all but entirely cease, and the blastoderm 
 remains inactive until, under the influence of the higher tem- 
 perature of natural or artificial incubation, the vital activities 
 of the germ are brought back into play, the arrested changes 
 go on again, and usher in the series of events which we have 
 now to describe in detail. 
 
 The condition of the blastoderm at the time when the 
 egg is laid is not exactly the same in all eggs, in some the 
 changes being farther advanced than in others, though the 
 differences of course are slight ; in some eggs, especially in 
 warm weather, changes of the same kind as those caused by 
 actual incubation may take place, to a certain extent, in the 
 interval between laying and incubation ; lastly, in all eggs, 
 both under natural and especially under artificial incubation, 
 the dates of the several changes are, within the limits of 
 some hours, very uncertain, particularly in the first few days ; 
 one egg being found, for example, at 36 hours in the same 
 stage as another at 24 or 30 hours, or a third at 40 or 48 
 hours. When we speak therefore of any event as taking 
 place at any given hour or part of any given day, we are to 
 be understood as meaning that such an event will generally 
 be found to have taken place at about that time. We 
 introduce exact dates for the convenience of description. 
 
44 THE FIRST DAY. [CHAP. 
 
 The changes which take place during the first day will be 
 most easily considered under three periods : from the 1st to 
 the 12th, from the 12th to the 20th, and from the 20th to the 
 24th hour. 
 
 2. From the 1st to about the 1.2th hour. During this 
 period the blastoderm when viewed from above is found to 
 have increased greatly in size. The pellucid area, which at 
 the best is but obscurely marked in the unincubated egg, 
 becomes very distinct (the central opacity having dis- 
 appeared), and contrasts strongly with the opaque area, 
 which has even still more increased both in distinctness 
 and size. 
 
 For the first few hours both the pellucid and opaque 
 areas remain circular, and the only change, besides increase 
 in size and greater distinctness which can be observed in 
 them, is a slight ill-defined opacity or loss of transparency, 
 which makes its appearance in about the middle of the 
 pellucid area. This is known as the embryonic shield. 
 
 3. Slight as are the changes which can at this stage be 
 seen from surface views, sections taken from hardened 
 specimens bring to light many most important changes in 
 the nature and arrangement of the constituent cells. 
 
 It will be remembered that the blastoderm in the un- 
 incubated egg is composed of two layers, an upper (Fig. 3, ep.) 
 and an under layer; that the upper is a coherent membrane 
 of columnar nucleated cells, but that the lower one (Fig. 3, 1.) 
 is formed of an irregular network of larger cells in which 
 the nuclei, if present, are rarely visible ; and that in addition 
 to this there are certain still larger cells, called ' formative 
 cells ' (Fig. 3, 6), lying at the bottom of the segmentation- 
 cavity. 
 
 Under the influence of incubation changes take place 
 very rapidly, which result in the formation of the three 
 layers of the blastoderm. 
 
 The upper layer, which we shall henceforward call the 
 epiblast (Fig. 10, A), takes but little share in these changes. 
 
 In the lower layer, however, certain of the cells begin 
 to get flattened horizontally, their granules become less 
 numerous, and a distinct nucleus makes its appearance in 
 them; the cells so altered cohere together and form a mem- 
 brane (Fig. 10, (7). The membrane thus formed, which is 
 
III.] THE MESOBLAST. 45 
 
 first completed in the centre of the pellucid area, we shall 
 henceforward speak of as the hypoblast. 
 
 Between it and the epiblast many of the cells of the 
 original lower layer are enclosed, and in addition some of the 
 formative cells (migrating by help of amoeboid movements 
 after the fashion of white-blood corpuscles) begin to travel 
 round the edge of the hypoblast, and to pass in between it 
 and the epiblast. 
 
 The cells, whether originally "formative" cells or cells 
 from the lower layer, thus gathered between the epiblast and 
 hypoblast, undergo a process of endogenous cell-formation, by 
 which the whole of the interior of each becomes converted 
 into a number of new cells, These new cells, spherical in 
 form, and possessing a large nucleus with a distinct nucleolus, 
 are first formed in the centre of the pellucid area and sub- 
 sequently in its periphery. They constitute the third layer 
 or mesoUast (Fig. 10, E). 
 
 The epiblast is the Horriblatt (corneal layer), and the hypoblast the Darm- 
 drusenblatt (epithelial glandular layer) of the Germans, while those parts of the 
 mesoblast which take part in the formation of the somatopleure and splanchno- 
 pleure correspond respectively to the Haut-muskd-platte and Darm-faser-platle. 
 
 All blood-vessels arise in the mesoblast. Hence the vascular layer of the 
 older writers fall entirely within the mesoblast. 
 
 The serous layer of the same authors includes the whole of the epiblast, but 
 also comprises a certain portion of mesoblast; for they speak of all the organs of 
 animal life (skin, bones, muscle, &c.) as being formed out of the serous layer, 
 whereas the epiblast proper gives rise only to the epidermis and to certain 
 parts of the nervous system. In the same way their mucous layer corresponds 
 to the hypoblast with so much of the mesoblast as takes part in the formation of 
 the organs of organic life. Their vascular layer therefore answers to a part only 
 of the mesoblast, viz. that part in which blood-vessels are especially developed. 
 
 It is worthy of notice that the cells of the epiblast are themselves the 
 direct results of segmentation ; but that the hypoblast and mesoblast are 
 formed at a subsequent period, and are therefore only indirectly the results of 
 segmentation. The true difference between the hypoblast and mesoblast lies 
 in the mode in which each layer is formed, and not in any essential differ- 
 ence in the segmentation-spheres from which each is derived. 
 
 At about the time when the hypoblast is completely 
 formed as a distinct membrane, the mesoblast cells form a 
 somewhat thick mass in the centre of the blastoderm, and 
 cause the central opacity spoken of above as the embryonic 
 shield. 
 
 4. Soon after this, between the 8th and 12th hours, the 
 hitherto circular pellucid area becomes oval (the opaque 
 area remaining circular) . The oval is, with remarkable regu- 
 
4G 
 
 THE FIRST DAY. 
 
 [CHAP. 
 
 larity, so placed that its long axis forms a right angle, or 
 very nearly a right angle, with the long axis of the egg 
 itself. Its narrow end corresponds with the future hind end 
 of the embryo : and we may henceforward speak of it as the 
 hind end. If an egg be placed with its broad end to the 
 right hand of the observer, the head of the embryo will in 
 nearly all cases be found pointing away from him. 
 
 At about the time when the pellucid area is beginning 
 to undergo this change of shape, there appears in surface 
 views, along a line corresponding with the long axis of 
 the oval, and occupying not, as might perhaps be expected, 
 its front but its hinder two-thirds, a narrow opaque streak, 
 much more opaque, and therefore distinct, than the em- 
 bryonic shield, but still shadowy and ill-defined. This is 
 known as the primitive streak. 
 
 FIG. 10. 
 
 
 SECTION OF A BLASTODERM AT RIGHT ANGLES TO THE LONG AXIS OF TEE 
 
 EMBRYO AFTER EIGHT HOURS' INCUBATION. 
 (Taken about midway between front and hind end.) 
 
 A. epiblast. B. mesoblast. C. hypoblast. pr. primitive groove. /. fold in 
 
 the blastoderm, probably produced by the action of the chromic acid. 
 
 m. c. mesoblast cell; the line points to one of the peripheral mesoblast 
 
 cells lying between epiblast and hypoblast. Id. formative cells. 
 
 The following are the chief points represented in the section, (i) The 
 
 thickening of the mesoblast underneath the primitive groove pr., even when 
 
 it is hardly at all present at the sides of the groove. (2) The hypoblast, C, early 
 
 formed as a single layer of spindle-shaped cells. (3) The so-called segmentation - 
 
 cavity, in which coagulated albumen is present. On the floor of this are the 
 
 large formative cells bd. 
 
 The line of separation between the epiblast and mesoblast underneath the 
 primitive groove is too strongly marked in the figure. 
 
 The primitive streak is no sooner formed than it becomes 
 marked on its upper surface by a delicate shallow furrow 
 
III.] THE PRIMITIVE GROOVE. 47 
 
 running along its axis. In fresh specimens viewed with 
 transmitted light, this furrow appears as a linear trans- 
 parency, but in hardened specimens seen under reflected 
 light may be distinctly recognized as a furrow or narrow 
 groove, the bottom of which being thinner than the sides 
 appears more transparent when viewed with transmitted 
 light. It is known as the primitive groove. The nature of 
 the changes by which it is brought about can only be learnt 
 by the study of vertical sections (Fig. 10). These teach us 
 that the opacity which marks out the primitive streak is 
 chiefly due to a thickening of the mesoblast. In the for- 
 mation, however, both of the primitive streak, and especially 
 of the primitive groove, the epiblast also plays an important 
 part. 
 
 During these twelve hours the epiblast has been spreading 
 rapidly, much more rapidly than the other two layers. Over 
 the white yolk in the region of the opaque area it forms a 
 layer one cell deep, but at the same time has become two or 
 three cells deep in the centre of the pellucid area. In the 
 pellucid area its constituent cells have become narrower (6/^) 
 and more columnar, but over the opaque area flatter and 
 broader (12 //,) than they were at first. At the 12th hour 
 therefore we find a distinct histological difference between 
 the epiblast cells of the pellucid and those of the opaque 
 area. 
 
 Over the thickening of the mesoblast, which forms the 
 basis of the primitive streak, the epiblast is also thickened ; 
 the hypoblast, however, remains here, as in the rest of the 
 blastoderm, a flat sheet consisting of a single layer of flat- 
 tened (seen in sections as a single row (Fig. 10, G) of spindle- 
 shaped) cells, which become larger ^nd more irregular at the 
 periphery. The thickening of the mesoblast and epiblast 
 in the region of the primitive streak causes the upper 
 outline of the blastoderm as seen in sections to rise above 
 the general surface in a gentle curve (Fig. 10). 
 
 The primitive groove is formed almost entirely by a 
 pushing in or depression of the epiblast at the summit of 
 this curve. 
 
 The thickness of the epiblast remains about the same on the sides as at the 
 bottom of the groove. The mesoblast, on the contrary, is thinner immediately 
 beneath the bottom of the groove than at the two sides, where it is decidedly 
 
48 THE FIKST DAY. [CHAP. 
 
 thicker than in the rest of the pellucid area. It is apparently this median 
 thinning of the mesoblast which gives rise to the linear transparency seen in 
 specimens viewed with transmitted light. The hypoblast, it may be remarked, 
 is generally curved downwards beneath the primitive streak and groove, though 
 not to the same extent as the epiblast. Thus the whole blastoderm is some- 
 what curved in this region. Immediately beneath the groove a kind of fusion 
 takes place between the epiblast and mesoblast, though on close examination 
 the line of junction between them can generally be made out. "This apparent 
 fusion His (Ueber die Erste Anlage des Wirbeltheirleibs) regarded as aw event 
 of great importance, and gave the name of axis-cord to the part in which it 
 occurs. In fresh specimens a narrow (opaque) streak can be seen running 
 down the centre of the groove; but it is not represented by any structure 
 which can be seen in sections. 
 
 The chief events then which occur during the first twelve 
 hours of incubation are the establishment of the three layers 
 of the blastoderm, and the appearance of the embryonic 
 shield, of the primitive streak and of the primitive groove. 
 
 5. From the 12th to the 20th hour. During this period 
 the pellucid area rapidly increases in size, and from being 
 oval becomes pear-shaped. The primitive groove grows even 
 more rapidly than the pellucid area; so that by the 16th hour 
 it is not only absolutely, but also relatively to, the pellucid 
 area, longer than it was at the 12th hour. . ; The interval 
 between its end and the circumference of the pellucid area 
 continues to be greater in front than behind. 
 
 At about the 16th hour, or a little later, a thickening of 
 the mesoblast takes place in front of the primitive groove, 
 giving rise to an opaque streak ending abruptly in front 
 against a semicircular fold, which appears at this time near 
 the anterior extremity of the pellucid area (Fig. 11), and 
 is known as the head-fold. In fresh specimens this streak 
 looks like a continuation from the anterior extremity of the 
 primitive groove ; but in hardened specimens it is easy to 
 see that the connection is only an apparent one. 
 
 Along the new streak a groove (Fig. 11, m. c.) is very soon 
 formed, which, narrow in front, but widening very much 
 behind, embraces between its diverging walls the anterior 
 extremity of the primitive groove. This new groove, by the 
 conversion of which into a tube the medullary canal will be 
 formed, is known as the medullary groove. 
 
 On each side of it the mesoblast is thickened, and the 
 surface of the blastoderm raised up in the form of two longi- 
 tudinal folds, known as the lamince dorsales, or the medullary 
 folds (Fig. 11, -4). Immediately beneath the bottom of the 
 
III.] 
 
 THE NOTOCHORD. 
 Fia. ii. 
 
 SURFACE VIEW OF THE PELLUCID AREA OF A BLASTODERM OF 18 HOURS. 
 
 None of the opaque area is shewn, the pear-shaped outline indicating the 
 limits of the pellucid area. 
 
 At the hinder part of the area is seen the primitive groove pr., with its 
 nearly parallel walls, fading away behind, but curving round and meeting in 
 front so as to form a distinct anterior termination to the groove, about half way 
 up the pellucid area. 
 
 Above the primitive groove is seen the medullary groove m. c., with the 
 medullary folds A. These diverging behind, slope away on either side of the 
 primitive groove, while in front they curve round and meet each other close 
 upon a curved line which represents the head-fold. 
 
 The second curved line in front of and concentric with the first is the com- 
 mencing fold of the amnion. 
 
 groove, however, the mesoblast is thinned out and very soon 
 the cells in this position, separating from the lateral masses, 
 adhere together in the middle line, and thus form between 
 the epiblast and the hypoblast a flattened circular rod known 
 as the notochord, seen in section as an elliptical aggregation 
 of cells (Fig. 12, cA.) 
 
 E. 4 
 
50 *THE FIRST DAY. [CHAP. 
 
 The medullary groove differs in many important particulars from the primi- 
 tive groove. Beneath the primitive groove the mesoblast always fuses more 
 or less with the epiblast ; this is never the case under the medullary groove. 
 Under the primitive groove the mesoblast never shews any signs of differ- 
 entiation into any organ ; under the medullary groove the notochord is formed 
 out of the mesoblast cells. The epiblast lining the bottom of the medullary 
 groove frequently becomes very much thinner than at its sides ; this seems 
 never to be the case with the primitive groove. 
 
 The primitive groove reaches its maximum growth before the appearance 
 of the medullary groove ; and after the appearance of the latter gradually 
 becomes less and less conspicuous, and finally disappears without leaving a 
 trace. A curved remnant of it is to be tound at the hind end of the medul- 
 lary canal between the 3oth and 4Oth hours, but by the 5Oth not a trace of it 
 remains. 
 
 By the earlier observers the primitive groove was supposed to become con- 
 verted into the medullary canal. Dursy (Der Primitivstreif des Hiihnchens) 
 was the first to give a correct account of its disappearance; and the distinction 
 between it and the medullary groove has since been fully recognized by many 
 observers. Goette (Archiv. Micr. Anat. Vol. x. 1873, PP- J 45 f 99) describes 
 the medullary groove as always appearing to the left of the primitive groove, 
 and having its floor continuous with the left wall of the latter. He states that 
 beneath this left wall the unsymmetrically placed axis-cord is found ; indeed 
 he considers that the notochord is a forward continuation of the axis-cord, and 
 that the latter, as the primitive groove recedes before the medullary groove, 
 becomes continuously converted into the former. 
 
 The primitive groove then is a structure which appears early, and soon 
 disappears without entering directly into the formation of any part of the 
 future animal. Apparently it has no function whatever. We can only sup- 
 pose that it is the rudiment of some ancestral feature. 
 
 6. By the 20th hour the medullary groove or canal ; with 
 its medullary folds or laminae dorsales, is fully established. It 
 then presents the appearance, towards the hinder extremity 
 of the embryo, of a shallow groove with sloping diverging 
 walls which embrace between them the remains of the 
 vanishing primitive groove. 
 
 Passing forwards towards what will become the head of 
 the embryo the groove becomes narrower arid deeper with 
 steeper walls. On reaching the head-fold (Fig. 11), which 
 continually becomes more and more prominent, the medul- 
 lary folds curve round and meet each other in the middle 
 line, so as to form a somewhat rounded end to the groove. 
 In front therefore the canal does not become lost by the 
 gradual flattening and divergence of its walls as is the case 
 behind, but has a definite termination, the limit being 
 marked by the head-fold. 
 
 In front of the head-fold, quite out of the region of 
 the medullary folds, there is usually another small fold 
 which is the beginning of the amnion (Fig. 11), 
 
III.] THE HYPOBLAST. 51 
 
 We must now go back, and say a few words about the 
 changes which the cells of the various layers undergo from 
 the 12 20 hours. 
 
 FIG. 12. 
 
 f/i, 
 
 TRANSVERSE SECTION OF A BLASTODERM INCUBATED FOR 18 HOURS. 
 
 The section passes through the medullary groove me., at some distance 
 behind its extreme front, and shews some of the chief points in which it differs 
 from the primitive groove. 
 
 The chief of these are, (i) the presence underrieath it of the notoohord ch. 9 
 (2) the absence of any apparent adhesion between the epiblast and the meso- 
 blast, (3) the thickening of the mesoblast underrieath the medullary folds, mf. 
 
 A. epiblast. B. mesoblast. C. hypoblast. 
 
 m. c. medullary groove. nt< /. medullary fold. ch. notochord : the small group 
 of mesoblast cells separated by a narrow gap from the thicker mass of 
 mesoblast on either side. 
 
 It is to be noticed that the cells of the hypoblast become more columnar as 
 they approach the edge of the pellucid area, and finally pass, without any strong 
 line of demarcation, into the white-yolk spheres. 
 
 Only one half of the section is represented if completed the section would 
 be symmetrical about the line passing through the centre of the medullary 
 canal, me. , 
 
 The hypoblast (Fig. 12, C) continues to be only one cell 
 deep ; the cells being, during the whole of this period, flatter 
 in the centre, and larger and more irregular towards the peri- 
 phery of the blastoderm. At about the 12th hour they are 
 very irregular in size ; shewing very great variations over 
 a very small space. This probably implies that they are 
 rapidly undergoing division. Later, however (about the 
 18th hour), they are fairly uniform over particular regions, 
 though they vary considerably in size at different parts of 
 the pellucid area. In no case does the hypoblast extend 
 beyond the edge of the pellucid area. 
 
 The hypoblast cells along the central axis of the pellucid area, and for some 
 little distance on each side, are smaller than elsewhere over the blastoderm. 
 Over a small district just outside the embryo, and at about one-third of the 
 way from the posterior extremity of the blastoderm, they are, from the i8th 231x1 
 hour, considerably larger than anywhere else. The remaining hypobiast cells 
 
 42 
 
52 THE FIRST DAY. [CHAP. 
 
 are intermediate in size between these very large cells and the smaller cells 
 in the centre. During the whole of this period the hypoblast cells continue 
 to be granular and filled with highly refractive spherules, exhibiting in this 
 respect a marked contrast to their appearance at a later time. 
 
 Their mode of increase is partly by division, but the layer grows chiefly 
 in a manner which is very different and somewhat remarkable. Before the 
 1 2th hour the hypoblast at its margin ended abruptly against the white-yolk 
 cells; but after that hour its relation to the white yolk becomes altered. As 
 they approach the white yolk the cells of the hypoblast become more and 
 more filled with white-yolk spherules, and at the extreme edge of the pellucid 
 area it is very difficult to say where the white yolk ends, and where the 
 hypoblast begins. This is somewhat diagrammatically shewn in Fig. 12. The 
 white-yolk spheres near the edge of the pellucid area have generally acquired 
 nuclei, though it is frequently difficult to see them owing to the numerous 
 highly refractive spherules which the spheres contain. The nearer they are 
 to the edge of the pellucid area the fewer spherules they contain, and at the 
 very edge it is almost impossible to say whether they ought to be called white- 
 yolk spheres or hypoblaat cells. The chief increase of the hypoblast therefore 
 seems to take place through the conversion, cell for cell, of the white yolk 
 into the hypoblast. 
 
 During this period the mesoblast (Fig. 12, J?) cells do 
 not undergo any marked change. The layer itself enlarges 
 to a certain extent through the multiplication of cells by 
 the division of old ones; but the chief increase in bulk is 
 probably due to the formative cells, which are continually 
 passing round from the bottom of the segmentation-cavity 
 to the mesoblast, and there become converted, in the way 
 described ( 3) above, into mesoblast cells. 
 
 These formative cells are more numerous at the bottom of the segmentation- 
 cavity at the 18th hour than they were at the first hour. This accession to 
 their number is probably due to fresh ones being formed from the floor of white 
 yolk. They appear to grow in size by absorbing the white-yolk spherules, 
 with which indeed they are completely filled. 
 
 The epiblast cells (Fig. 12, A) probably increase entirely 
 by division, and seem to derive their nourishment from the 
 white yolk on which the peripheral cells rest, and perhaps 
 also from the albuminous fluid which fills the segmentation- 
 cavity and occupies all the interstices between the cells of 
 the various layers. 
 
 The cells near the edge of the opaque area are the largest and flattest 
 of the epiblast cells; those in the middle of the pellucid area are smaller than 
 those at its edge. 
 
 Outside the blastoderm there are to be seen on the surface of the yolk 
 alternating transparent and opaque white rings. These are known as the 
 halones, and frequently appear at the commencement of incubation. It is 
 stated by His that they are to be explained by the white-yolk spheres under- 
 going changes of two kinds. In the one case the spherules they contain are 
 
III.] THE MEDULLARY CANAL. 53 
 
 dissolved and give place to vacuoles ; where this occurs to a large extent an 
 opaque ring is formed. In the other case a solution of the protoplasm of the 
 spheres takes place, and the spherules are let loose in large numbers ; where 
 this occurs a transparent ring is formed. 
 
 The chief events then of the second part of the first day, 
 are the appearance of the medullary folds and groove, the 
 formation of the notochord, the beginning of the head-fold 
 and amnion, and the histological changes taking place in the 
 several layers. 
 
 7. From the 20th to the 24<th hour. The head-fold en- 
 larges rapidly, the crescentic groove becoming deeper, while 
 at the same time the overhanging margin of the groove (the 
 upper limb of the 8, Chap. II. 5), rises up above the level of 
 the blastoderm ; in fact, the formation of the head of the 
 embryo may now be said to have definitely begun. 
 
 S. The medullary folds, increasing in size in every di- 
 mension, but especially in height, lean over from either side 
 towards the middle line, and thus tend more and more to 
 roof in the medullary canal, especially near the head. 
 About the end of the first day they come into direct contact 
 and completely coalesce with each other at a point which 
 lies at some little distance behind the head-fold, in the 
 region which will afterwards become the neck. Union, having 
 begun at this spot, rapidly runs forward till (early in the 
 second day) the head-part is completely closed in ; and then 
 passes more slowly backwards. The whole of the anterior 
 portion of the groove is closed in before the union has ad- 
 vanced more than a very short distance towards the tail. In 
 this way a tubular canal is formed, ending blindly in front, 
 but as yet open behind. This is the medullary or neural 
 canal (Fig. 13, M, Fig. 20, Me.). It is not completely 
 closed in at the tail till a period considerably later than 
 the one we are considering. 
 
 9. Meanwhile important changes are taking place in 
 the axial portions of the mesoblast, which lie on each side of 
 the notochord beneath the medullary folds. 
 
 In an embryo of the middle period of this day, examined 
 with transmitted light, the notochord is seen at the bottom 
 of the medullary groove between the medullary folds, as a 
 transparent line shining through the floor of the groove when 
 the embryo is viewed from above. On either side of the 
 
54 THE FIRST BAY. [CHAP. 
 
 notochord the body of the embryo appears somewhat opaque, 
 owing to the thickness of the medullary folds ; as these folds 
 slope away outwards on either side, so the opacity gradually 
 fades away in the pellucid area. There is present at the sides 
 no sharp line of demarcation between the body of the embryo 
 and the rest of the area ; nor will there be any till the lateral 
 folds make their appearance ; and transverse vertical sections 
 shew (Fig. 12) that there is no break in the mesoblast, from 
 the notochord to the margin of the pellucid area, but only a 
 gradual thinning. 
 
 10. During the latter period of the day, however, the 
 plates of mesoblast on either side of the notochord begin to 
 be split horizontally into two layers, the one of which attach- 
 ing itself to the epi blast, forms with it the somatopleure (Fig. 
 13, compare also Fig. 20, So.}, while the other, attaching itself 
 to the hypoblast, forms with it the s/jlanchnopleure (Fig. 13, 
 Be, Fig. 20, sp). By the separation of these two layers from 
 each other, a cavity (Fig. 13, pp, and Fig. 20, pp], containing 
 fluid only, and more conspicuous in certain parts of the 
 embryo than in others, is developed. This cavity is the be- 
 ginning of that great serous cavity of the body which after- 
 wards becomes divided into separate cavities. We shall speak 
 of it as the pleuro-peritoneal cavity. 
 
 11. This cleavage into somatopleure and splanchnopleure 
 does not extend quite up to the walls of the medullary canal. 
 Hence there is left along either side of the canal, between it 
 and the line along which the cleavage begins, a tract or plate 
 of uncleft mesoblast, which receives the name of vertebral 
 plate, the more external mesoblast being called the lateral 
 plate. 
 
 At first each vertebral plate is not only unbroken along 
 its length but also continuous at its outer edge with the 
 upper and lower layers of the lateral plate of the same side. 
 Very soon, however, clear transverse lines are seen, in surface 
 views, stretching inwards across each vertebral plate from the 
 lateral plate towards the notochord ; and not long after a 
 transparent longitudinal line makes its appearance on either 
 side of the notochord along the line of junction of the lateral 
 with the vertebral plate. 
 
 These transparent lines are caused by the appearance of 
 vertical clefts, giving rise to narrow spaces containing nothing 
 
III.] 
 
 THE PROTOVERTEBR.E. 
 
 55 
 
 but clear fluid ; and transverse sections shew that they are 
 due to breaches of continuity in the mesoblast only, the 
 epiblast and hypoblast having no share in the matter. The 
 first transverse lines which appear are two in number, one a 
 little behind the other, about opposite the spot where the 
 medullary folds first coalesced to form the neural tube. The 
 longitudinal lines begin at about the same place and run thence 
 backward, parallel to the notochord, as far as the closure of 
 the medullary canal extends. Behind the first two transverse 
 lines other parallel transverse lines in course of time make 
 their appearance. 
 
 Thus each vertebral plate appears in surface views to be 
 cut up into a series of square plots, bounded by transparent 
 lines. Each square plot is the surface of a corresponding 
 cubical inass {Fig. 13, P. v., Fig. 20, P. v.). The two such 
 
 FIG. 13, 
 
 
 \Ch 
 
 'Ao 
 
 TRANSVERSE SECTION THROUGH THE DORSAL KEG ION OF AN~ EMBRYO OP THE 
 SECOND DAT. (Copied from His), introduced here to illustrate the forma- 
 tion of the protovertebrse, and the cleavage of the mesoblast. 
 
 The provertebrse appear irregularly quadrate ; they would have been more 
 distinctly square, had the section been one of the fi' - st day, before the appear- 
 ance of the primitive aortse, and of the rudiments of the Wolffiari ducts : com- 
 pare Fig. 20. 
 M. medullary canal. Pv. protovertebra. w. rudiment of Wolffian duct. 
 
 A. epiblast. C. hypoblast. Ch. notochord. Ao. aorta. BC. splanch- 
 
 nopleure. 
 
 cubical masses first formed, lying one on either side of the 
 notochord beneath, and a little to the outside of the medul- 
 lary folds, are the first pair of protovertebrce. Behind this 
 first pair, but otherwise similarly situated, a second and third 
 pair make their appearance during the first day. 
 
56 THE FIRST DAY. [CHAP. 
 
 The vertebral plate, while still continuous with the lateral plate (the dis- 
 tinction between the two being indicated solely by the cleavage of the latter), 
 consists of several layers of cells ; but of these only the uppermost layer, that 
 immediately under the epiblast, appears to be continued into the somatopleure ; 
 the whole of the remainder, including those cells which will eventually form the 
 so-called nucleus of the protovertebrae, seem to pass directly into the splanchno- 
 pleure. 
 
 All these changes except the formation of the pleuro-peri- 
 toneal cavity can be seen in surface views of fresh trans- 
 parent specimens, but their nature is best shewn in sections. 
 
 12. Since the commencement of incubation the area 
 opaca has been spreading outwards over the surface of the 
 yolk, and by the end of the first day has reached about the 
 diameter of a sixpence. It appears more or less mottled 
 over the greater part of its extent, but this is more particu- 
 larly the case with the portion lying next to the pellucid 
 area ; so much so, that around the pellucid area an inner ring 
 of the opaque area may be distinguished from the rest by 
 the difference of its aspect. 
 
 At about the 20th 24th hours an increasing number of 
 formative cells make their way from the segmentation-cavity 
 to the edge of the area opaca, and there, immediately under- 
 neath the epiblast, quickly become converted into a rather 
 thick and somewhat irregular network of mesoblast cells. The 
 mottled appearance of the inner ring spoken of above is due 
 to changes taking place in this mass of mesoblast, changes 
 which eventually result in the formation of what is called the 
 vascular area, the outer border of which marks the extreme 
 limit to which the mesoblast extends. 
 
 During the whole of this period the medullary groove has 
 been growing rapidly backwards, so that the primitive groove 
 appears to be pushed further and further back, and at the 
 same time becomes smaller and less conspicuous. The 
 amniotic fold is at the end of the first day very noticeable. 
 
 13. The changes then which occur during the first day 
 may thus be briefly summarized : 
 
 (1) The hypoblast and mesoblast are formed from the 
 segmentation-spheres, so that by the 6th to the 8th hour the 
 three layers of the germ the epiblast , the mesoblast , and the 
 hypoblast are definitely established. 
 
 (2) The primitive streak is formed by a thickening of 
 the mesoblast. 
 
III.] SUMMARY. 57 
 
 (3) The primitive groove is formed along the centre of 
 the primitive streak. 
 
 (4) The pellucid area becomes pear-shaped, the broad 
 end corresponding with the future head of the embryo. Its 
 long axis lies at right angles, to the long axis of the egg. 
 
 (5) The medullary groove makes its appearance in front 
 of the primitive groove, and below it the notochord is formed 
 out of mesoblastic cells. 
 
 (6) The development of the head-fold gives rise to the 
 first definite appearance of the head. 
 
 (7) The medullary folds rise up and coalesce in the 
 region of the neck to form the neural tube, the primitive 
 streak and groove disappearing. 
 
 (8) One or more pair of protovertebrce make their ap- 
 pearance. 
 
 (9) By the cleavage of the mesoblast, the somatopleure 
 separates from the splanchnopleure. 
 
 (10) The first trace of the amnion appears in front of 
 the head-fold. 
 
 (11) The vascular area begins to be be distinguished 
 from the rest of the opaque area. 
 
 It may be well to remark, before passing on to the second 
 day, that out of the protovertebras are formed not only the 
 permanent vertebrae, but also the superficial dorsal as well 
 as certain other muscles and the spinal nerves ; that the pair 
 of protovertebras first formed corresponds not with the first 
 cervical vertebra of the adult chick, but rather with the third 
 or even fourth ; for though the majority of the protovertebra3 
 are formed regularly behind the first pair, two or even three 
 pair may make their appearance in front of it; and lastly, 
 that in the part of embryo which forms the head, the meso- 
 blast is never cut up into proto vertebras, and never under- 
 goes cleavage to form somatopleure and splanchnopleure. 
 
CHAPTER IV. 
 
 THE CHANGES WJIICH TAKE J>I,ACE DURING THE SECOND 
 
 DAY. 
 
 1. The First Half of the Second Day. In attempting 
 to remove the blastoderm from an egg which has undergone 
 from 30 to 36 hours' incubation, the observer cannot fail. to 
 notice a marked change in the consistency of the blastodermic 
 structures. The excessive delicacy and softness of texture 
 which rendered the extraction of an 18 or 20 hours' blasto- 
 derm so difficult, has given place to a considerable amount of 
 firmness; the outlines of the embryo and its appendages are 
 much bolder and more distinct ; and the whole blastoderm 
 can be removed from the egg with much greater ease, 
 
 In the embryo itself viewed from above one of the fea- 
 tures which first attracts attention is the progress in the 
 head-fold (Fig. 15). The upper limb or head has become 
 much more prominent, while the lower groove is not only 
 proportionately deeper, but is also being carried back beneath 
 the body of the embryo (Chap. n. 5.) 
 
 2. The medullary folds are closing rapidly. In the region 
 of the head they have quite coalesced, a slight notch in the 
 middle line at the extreme front marking for some little time 
 their line of junction. The open medullary groove of the first 
 day has thus become converted into a tube, the neural canal, 
 closed in front, but as } 7 et open behind. For a brief period 
 the calibre of this tube is uniform throughout; but very 
 speedily the front end dilates into a small bulb, whose cavity 
 remains continuous with the rest of the neural canal, and 
 whose walls, like those of the canal, are formed of epiblast. 
 This bulb is known as the first cerebral vesicle, Fig. 14, FB, 
 
CHAP. IV.] 
 
 THE CEREBRAL VESICLES. 
 
 59 
 
 AN EMBRYO CHICK OF THE FIRST DAT (ABOUT THIRTY-SIX HOURS) VIEWED 
 
 FROM BELOW AS A TRANSPARENT OBJECT. 
 
 pi. Outline of the pellucid area. 
 
 FB. The forebrain or first cerebral vesicle, projecting from the sides of which 
 are seen the optic vesicles op.. A definite head is now constituted, the 
 backward limit of the somatopleure fold being indicated by the faint 
 line S. 0. Around the head are seen the two limbs of the amniotic head- 
 fold : one, the true amnion a, closely enveloping the head, the other, the 
 false amnion a', at some distance from it. The head is seen to project 
 beyond the anterior limit of the pellucid area. 
 
 The splanchnopleure fold extends as far back as sp. Along its diverging 
 limbs are seen the conspicuous venous roots or omphalo-mesaraic veins, uniting 
 to form the heart h, which continuing forward as the bulbus arteriosus ba, is 
 lost in the substance of head just in front of the somatopleure fold. 
 
 Lying (in this position of the embryo) under the heart is seen the broad 
 foregut d, the wide crescentic opening into which at the hind limit of the 
 splanchnopleure fold is very conspicuous. Beneath the foregut are faintly seen 
 the hind brain tiB. and higher up and more distinctly the mid brain MB. 
 These are not yet completely differentiated, and their limits are in consequ ence 
 very obscurely indicated. 
 
GO THE SECOND DAY. [CHAP. 
 
 Behind the splanchnopleure fold, marking the hind limits of the foregut, 
 are seen the two rows of proto vertebrae, the dark line between which m. c. 
 indicates the position both of the line of junction of the medullary folds and of 
 the notochord. The front end of the notochord is seen at ch. underneath the 
 forebrain; its hind end is indistinct. Towards the tail the proto vertebrae 
 become indistinct and give place to the vertebral plates v. pi. Still further 
 back, at the commencing tail, all the parts become indistinct, the remains of 
 the primitive groove pv. being as conspicuous as anything else. 
 
 and makes its appearance in the early hours of the second 
 day. Behind it a second and a third bulb, the second and 
 third cerebral vesicles, are successively formed in a similar 
 manner; but the consideration of these, though they begin 
 to make their appearance soon after the formation of the 
 first cerebral vesicle, may be conveniently reserved to a later 
 period. 
 
 3. The number of proto vertebras increases rapidly. 
 The one or two pairs which are seen at the end of the first 
 day have by the middle of the second day multiplied to five, 
 or eight, or even more, Figs. 14, 15, p.v, each being formed in 
 the same way as the first. As was mentioned previously, 
 the chief increase takes place from before backwards, the 
 new protovertebras appearing behind the old ones; but one 
 pair at least is probably formed in front of that which was 
 the very first to appear. 
 
 In the early part of this day the formation of new proto- 
 vertebras keeps pace with the closing in of the medullary folds, 
 so that that part of the canal which is already closed in is 
 always flanked by protovertebras ; but later on the formation 
 of protovertebrge lags behind, so that for some distance to- 
 wards the hinder extremity the closed medullary canal is 
 unprotected by proto vertebras, Fig. 15. At the extreme end 
 the medullary folds become shallower, diverge from each 
 other, and afterwards meet again, thus forming a lozenge- 
 shaped open depression known as the sinus rhombpidalis, 
 Fig. 15, s. r. 
 
 Behind the sinus rhomboidalis there may generally be seen a small and 
 usually curved remnant of the primitive groove. Fig. 15, p. r. 
 
 4. In a former chapter it was pointed out (Chap. II. 5) 
 that the embryo is virtually formed by a folding or tucking 
 in of the limited portion of the blastoderm, first at the anterior 
 extremity, and afterwards at the posterior extremity and at 
 
IV.] 
 
 THE ALIMENTARY CANAL. 
 FIG. 15. 
 
 61 
 
 EMBRYO OF THE CHICK AT 36 HOURS VIEWED FROM ABOVE AS AN OPAQUE 
 
 OBJECT. (Chromic acid preparation.) 
 /. b. front brain, m. b. mid brain. Ti. b. hind brain, op. v. optic vesicle. 
 
 au. p. auditory vesicle, o. f. omphalo-mesaraic vein. p. v. protovertebra. 
 
 m. f. line of junction of the medullary folds above the medullary canal. 
 
 8. r. sinus rhomboidalis. t. tail-fold, p. r. remains of primitive groove. 
 
 a. p. area pellucida. 
 
 The line to the side between p. v. and m. f. represents the true length 
 of the embryo. 
 
 The biscuit-shaped outline indicates the margin of the pellucid area. The 
 head, which reaches as far back as o.f, is distinctly marked off ; but neither the 
 somatopleuric nor splanchnopleuric folds are shewn in the figure ; the latter 
 diverge at the level of o.f, the former considerably nearer the front, somewhere 
 between the lines m. b. and h. b. The optic vesicles op. v. are seen bulging 
 out beneath the superficial epiblast. The heart lying underneath the opaque 
 body cannot be seen. The tail-fold, t., is just indicated ; no distinct lateral 
 folds are as yet visible in the region midway between head and tail. At m. /. 
 the line of junction between the medullary folds is still visible, being lost 
 forwards over the cerebral vesicles, while behind the folds diverge to enclose 
 the narrowing sinus rhomboidalis, s. r. 
 
62 
 
 THE SECOND DAY. 
 
 [CHAP. 
 
 the sides. One of the results of this doubling up of the blasto- 
 derm to form the head is the appearance, below the anterior 
 extremity of the medullary tube, of a short canal, ending 
 blindly in front, but open widely behind (Fig. 16, D], a cut de 
 sac in fact, lined with hypoblast reaching from the extreme 
 front of the embryo to the point where the splanchnopleuric 
 leaf of the head-fold (Fig. 16, F. Sp) turns back on itself. 
 This cul de sac, which of course becomes longer and longer 
 the farther ba.ck the head-fold is carried, is the rudiment of 
 the front end of the alimentary canal, the foregut, as it might 
 be called. In transverse section it appears to be flattened 
 horizontally, and also bent, so as to have its convex surface 
 looking downwards, (Fig. 18 al). At first the anterior end is 
 quite blind, there being no mouth at all ; the formation of 
 this at a subsequent date will be described later on. 
 
 At the end of the first half of the second day the head- 
 
 FIG. 1 6. 
 
 N.C. 
 
 F.So. 
 
 DIAGRAMMATIC LONGITUDINAL SECTION THROUGH THE Axis OF AN EMBRYO. 
 The section is supposed to be made at a time when the head -fold has oom^ 
 menced but the tail-fold has not yet appeared. 
 
 N. C. neural canal, closed in front but as yet open behind. Ck. notochord, not 
 reaching to the extreme front, and not as yet fully formed behind. The 
 section being taken in the middle line, the protovertebrse are of course not 
 shewn. In front of the notochord is seen a mass of un cleft mesoblast, 
 which will eventually form part of the skull. D. the commencing fore- 
 gut or front part of the alimentary canal. F. So. Somatopleure, raised 
 up in its peripheral portion into the amniotic fold Am. Sp. Splanchno- 
 pleure. At Sp. it forms the under wall of the foregut; at F. Sp. it is turning 
 round and about to run forward. Just at its turning point the cavity of the 
 heart lit is being developed in its mesoblast. pp. pleuroperitoneal cavity. 
 A epiblast, B mesoblast, C hypoblast, indicated in the rest of the figure by 
 differences in the shading. At the part where these three lines of reference 
 end the mesoblast is as yet uncleft. 
 
IV.] THE HEART. 63 
 
 fold has not proceeded very far backwards, and its limits 
 can easily be seen in the fresh embryo both from above and 
 from below. 
 
 5. It is in the head-fold that the formation of the heart 
 takes place, its mode of origin being connected with that 
 cleavage of the mesoblast and consequent formation of splanch- 
 nopleure and somatopleure of which we have already spoken. 
 
 At the extreme end of the embryo (Fig. 16), where the 
 blastoderm begins to be folded back, the mesoblast is never 
 cleft, and here consequently there is neither somatopleure nor 
 splanchnopleure ; but at a point a very little further back, 
 close under the blind end of the foregut, the cleavage (at 
 the stage of which we are speaking) begins, and the somato- 
 pleure, F. So, and splanchnopleure, F. Sp. diverge from 
 each other. They thus enclose between them a cavity, pp, 
 which rapidly increases behind by reason of the fact that 
 the fold of the splanchnopleure is carried on towards the 
 hinder extremity of the embryo considerably in advance of 
 that of the somatopleure. Both folds, after running a certain 
 distance towards the hind end of the embryo, are turned 
 round again, and then course once more forwards over the 
 yolk-sac. As they thus return (the somatopleure having 
 meanwhile given off the fold of the ammion, Am.), they are 
 united again to form the uncleft blastodermic investment of 
 the yolk-sac. In this way the cavity arising from their sepa- 
 ration is closed below. 
 
 It is in this cavity, which from its mode of formation the 
 reader will recognise as a part (and indeed at this epoch it 
 constitutes the greater part) of the general pleuroperitoneal 
 cavity, -that the heart is formed. 
 
 It makes its appearance at the under surface and hind 
 end of the foregut just where the splanchnopleure folds turn 
 round to pursue a forward course, (Fig. 16, Ht)\ and by the 
 end of the first half of the second day (Fig. 14, h) has ac- 
 quired somewhat the form of a flask with a slight bend to 
 the right. At its anterior end a slight swelling marks the 
 future bulbus arUriosus ; and a bulging behind indicates the 
 position of the auricles. It is hollow, and its cavity opens 
 below and behind into two vessels called the omphalo-mesaraic 
 veins (Figs. 14, 15, o.f), which pass outwards in the folds of the 
 splanchnopleure at nearly right angles to the axis of the 
 
64 THE SECOND DAY. [CHAP. 
 
 embryo. The anterior extremity of the heart is connected 
 with the two aortse. 
 
 The muscular portion of the walls of the heart are derived in the chick (as 
 in all other vertebrates in which the point has been worked out) from the 
 mesoblast of the splanchnopleure. 
 
 Although thus much may be asserted with tolerable certainty for all verte- 
 brates; yet the exact mode of development appears, according to our present 
 knowledge, to be very different in different cases ; and it seems probable that 
 these differences are in part the result of variations in the mode of formation 
 and time of closure of the alimentary canal. 
 
 In the chick the investigation of the earlier stages of the heart is beset with 
 considerable difficulties; and accordingly various inquirers have arrived at very 
 different results, though the majority are agreed as to its formation from the 
 mesoblast of the splanchnopleure. Exact information concerning the epithe- 
 lium lining the heart may be said to be almost completely wanting. 
 
 Von Baer described the heart as consisting in its earliest stage of two solid 
 aggregations of the mesoblast cells of the splanchnopleure, converging in front 
 at the end of the foregut where they are loosely united together by a thin band, 
 but diverging behind along the diverging folds of the splanchnopleure. As the 
 foregut lengthens, the two masses coalesce more and more completely in front, 
 until the whole structure assumes the shape of a fusiform mass, attached to the 
 under wall of the foregut, with prolongations stretching like the limbs of 
 an inverted & along the folds of the splanchnopleure on either side. A t first 
 solid throughout, the ^-shape mass subsequently becomes hollowed out and 
 filled with fluid by the solution of its central cells. 
 
 The account given by Eemak (EntwicTcelung der Wirbelthiere, 1855) is some- 
 what similar. 
 
 According to His the heart is formed by the separation of a layer of the 
 splanchnopleure and its coalescence with a similar layer from the somatopleure. 
 It is therefore from the beginning hollow. Its cavity is also from the be- 
 ginning continuous with the canals of the aortae and omphalo-mesaraic veins, 
 the roots of which are formed in a precisely similar manner as itself. It is 
 through these that the epithelial (endothelial) elements, derived from the white 
 yolk, make their way into the heart to form its epithelial (endothelial) lining. 
 
 According to Afanassieff (Bull. Acad. St. Petersbourg., Tom. xm. 1869, 
 pp. 221 335) the heart is formed by the longitudinal separation of a thick 
 layer of the mesoblast of the splanchnopleure along the under wall of the fore- 
 gut. At either side, so much of the mesoblast is detached, that only a single 
 layer of cells (seen spindle-shaped in a transverse section of the embryo) remains 
 united with the hypoblast to form the wall of the gut. Along the middle line 
 the separation is not complete, the detached layer of mesoblast being here still 
 connected with the wall of the gut by a few cells. The single layer of spindle- 
 shaped cells breaks loose in turn from the wall of the gut on either side of the 
 mill die line, but still remains attached along the middle line itself. We have 
 thus in transverse section a thinner and a thicker layer of mesoblast, hanging 
 down in a double festoon from the (hypoblastic) under-wall of the gut. Both 
 layers become more and more separated from the gut, and bulge out into the 
 pleuroperitoneal space, thus creating between themselves and the gut a cavity, 
 which is at first double, but, by the disappearance of the cells along the middle 
 line, subsequently becomes single. This is the cavity of the heart, the thick 
 layer representing its muscular walls, and the thin its epithelial lining. The 
 two ends are open, the hinder end being connected with the omphalo-mesaraic 
 veins, and the front with the aortse. At first the heart is not a tube with 
 
IV.] THE HEART. 65 
 
 complete walls of its own, but rather a cavity, closed in below and at the sides 
 by its mesoblantic walls, and roofed over by the bare hypoblastic under-wall of 
 the foregut. Very shortly, however, the side walls close in above, and thus 
 pinch off the heart as a complete and distinct tube, which becomes quite 
 detached along the greater part of its length from the wall of the gut, though it 
 still remains connected with it, both at the venous and arterial ends. 
 
 Klein (Wien. Sitzungsbericht, LXIII. n., iSyr) considers that the heart 
 is formed from the cells of the mesoblast of the splanchnopleure as a body 
 which, at first solid, subsequently becomes hollow by the conversion of its 
 central cells into blood-corpuscles. The layer of cells immediately surrounding 
 the blood-corpuscles forms the epithelial lining, and subsequently becomes 
 connected with that of the great arteries and veins. 
 
 The following view, which our own observations have led us to adopt, 
 agrees with that of Klein in regarding the heart as being at first a solid thicken- 
 ing of the mesoblast of the splanchnopleure ; but its accordance with the earlier 
 Statements of Von Baer is much more complete. 
 
 In order to understand the formation of the heart it must be distinctly borne 
 in mind that in the region where the heart is about to appear, the splanch- 
 nopleure is continually being folded in on either side, and that these lateral 
 folds are progressively meeting and uniting in the middle line to form the 
 under-wall of the foregut (that which in the adult chick will be the anterior 
 wall of some portion of the alimentary canal). (Compare Chap. n. 5.) 
 At any given moment these folds will be found to have completely united 
 in the middle line along a certain distance measured from the point in front 
 where the cleavage of the mesoblast (i.e. the separation into somatopleure 
 and splanchnopleure) begins, to a particular point farther back. 
 
 At this particular point the folds will have met, but not united, Fig. 17, A. 
 Further back still they will have not even met, but will appear simply inclined 
 towards each other, Fig. 17, B. Or, to put it in another way, they will here be 
 found to be diverging from the point where they were united, and not only diverg- 
 ing laterally each from the middle line, but also both turning so as to run in 
 a forward direction to regain the surface of the yolk and rejoin the somato- 
 pleure, Fig. 1 6. In a transverse section taken behind this extreme point 
 of union, or point of divergence, as we may call it, the splanchnopleure on 
 either side when traced downwards from the axis of the embryo may be 
 seen to bend in towards the middle so as to approach its fellow, and then 
 to run rapidly outwards, Fig. 17, B. A longitudinal section shews that it runs 
 forwards also at the same time, Fig< 16. A section through the very point 
 of divergence shews the two folds meeting in the middle line and then separating 
 again, so as to form something like the letter X, with the upper limbs con- 
 verging, and the lower limbs diverging. In a section taken in front of the 
 point of divergence, Fig. 18, the lower diverging limbs of the X have disappeared 
 altogether; nothing is left but the upper limbs, v\hich, completely united in the 
 middle line, form the under- wall of the foregut. 
 
 As development proceeds, what we have called the point of divergence 
 is continually being carried farther and farther back, so that the distance 
 between it and the point where the somatopleure and splanchnopleure separate 
 from each other in front, i.e. the length of the foregut, is continually increasing. 
 
 When the heart is about to be formed, thickenings are observed in the 
 mesoblast of the splanchnopleure, along the diverging folds, i.e. along the 
 lower limbs of the X just behind the point of divergence. Ihese thickenings 
 are continued into each other by a similar thickening of the mesoblast ex- 
 tending through the point of divergence itself. 
 
 At first there is no thickening of the mesoblast in front of the point of 
 divergence, i.e. along the under- wall of the foregut. As the point of diverg- 
 
 E. 5 
 
06 THE SECOND DAY. [CHAP. 
 
 ence however is in the Course of events carried farther back, though the 
 lower diverging folds (the lower limbs of the X) disappear, the thickening at 
 the point remains and increases. In a short time, consequently, we do find 
 a thickening of the mesoblast in the under-wall of the foregut just in front 
 of the point of divergence, which thickerting is continuous like an inverted j^, 
 with two thickenings reaching down the diverging folds behind the point 
 of divergence. 
 
 This j^-shaped thickening becomes hollow by a transformation of its 
 central cells ; the single cavity in front is the cavity of the heart, and the two 
 diverging cavities behind, with which it is continuous, are the canals of the 
 omphalo-mesaraic veins. 
 
 As development proceeds, and the point of divergence is carried still farther 
 and farther back, the heart increases in length step by step at the expense 
 of the continually coalescing omphalo-mesaraic veins. 
 
 The coalescence of the mesoblastic thickening which forms the walls of the 
 veins precedes that of their canals, consequently in sections taken at parti- 
 cular points we meet with two cavities invested by one wall. This is probably 
 what was seen by the observers who have described the heart as being formed 
 as a double tube which afterwards became single. 
 
 The front end of the cavity of the heart is continuous with canals similarly 
 formed in the mesoblast of the foregut by the solution of certain ceils. These 
 are the canals of the aortse. 
 
 At first the substance of the heart is along its whole length adherent to and 
 indeed a part of the underwall of the foregut. Subsequently it becomes free in 
 its middle portion, the arterial and venous ends alone remaining attached. 
 
 Soon after its formation the heart begins to beat, its at 
 first slow and rare pulsations beginning at the venous and 
 passing on to the arterial end. It is of some interest to 
 note that its functional activity commences long before the 
 cells of which it is composed shew any distinct differentiation 
 into muscular or nervous elements. 
 
 6. To provide channels for the fluid thus pressed by the 
 contractions of the heart, a system of tubes has made its 
 appearance in the mesoblast both of the embryo itself and of 
 the vascular and pellucid areas. In front the single tube of 
 the heart bifurcates into two primitive aortce, each of which 
 bending round the front end of the foregut, passes from its 
 under to its upper side, the two forming together a sort of 
 incomplete arterial collar imbedded in the mesoblast of the 
 gut. Arrived at the upper side of the gut, they turn sharply 
 round, and run separate but parallel to each other backwards 
 towards the tail, in the mesoblast on each side of the notochord 
 immediately under the protovertebra (Figs. 18, Ao, 20, Ao). 
 About half way to the hinder extremity each gives off at right 
 angles to the axis of the embryo a large branch, the omphalo* 
 mesaraic artery (Fig. 23, Of, A.}, which, passing outwards, is 
 
IV.] 
 
 THE BLOOD-VESSELS. 
 
 67 
 
 distributed over the pellucid and vascular areas, the main 
 trunk of each aorta passing on with greatly diminished calibre 
 towards the tail, in which it becomes lost. 
 
 FIG. 17. A. 
 
 FIG. 17. B, 
 
 Tib, 
 
 <*.? 
 
 TWO CONSECUTIVE SECTIONS OF A 36 HOURS' EMBRYO ILLUSTRATING THE 
 FORMATION OF THE HEART. A IS THE MOST ANTERIOR OF THE TWO. 
 
 h. b. hind brain. nc. notochord. E. epiblast. so. somatopleure. sp. 
 splanchnopleure. d. alimentary canal, hy. hypoblast. hz. (in A) heart. 
 of. omphalo-mesaraic vein. 
 
 The heart is seen from the sections to be formed from the mesoblast of the 
 splanchnopleure. It is not however split off from a portion of the mesoblast 
 which forms the muscular wall of the alimentary canal, but the mesoblast, where 
 it turns round to run outwards again over the yolk-sack, becomes thickened, 
 and in each of the thickenings (one on each side) so formed, a cavity appears 
 
 52 
 
68 THE SECOND DAY. [CHAP. 
 
 forming immediately behind'the heart the omphalo-mesaraic veins (section J?), (of). 
 As however the folding of the splanchnopleure becomes more complete, and the 
 digestive canal becomes completely closed (instead of remaining partially open 
 as in section B), these two cavities unite; and an appearance is produced 
 similar to that represented in figure A, where there is the single cavity of the 
 heart (hz). In the interior of the heart is seen a lining of flattened cells. 
 
 The shading, as will be seen, is purely diagrammatic. The epiblast, 
 whether superficial as at E, or involuted as part of the neural canal hb, is 
 shaded of one tint. The mesoblast, whether uncleft, or diverging into soma- 
 topleure and splanchnopleure, is of another tint. In the hypoblast a distinction 
 has been drawn between the thickened portion which lines the alimentary 
 canal, and the thinner portion which belongs to the more peripheral part 
 of the splanchnopleure, the two being at first continuous as in B, and afterwards 
 separated as in A. 
 
 It will be understood that the two figures, though actually two consecutive 
 sections of the same embryo, may be taken to represent two phases of the 
 formation of the heart. B in the process of development will become A, and 
 A a short time previously was in the condition of B. 
 
 TRANSVERSE SECTION OF AN EMBRYO AT THE END OF THE SECOND DAY PASSING 
 
 THROUGH THE REGION OF BULBUS ARTEttlOSUS. (Copied from His.) 
 
 M. medullary canal in the region of the hind brain. V. anterior cardinal or 
 superior vertebral vein. Ao. Aorta. Ch. Notochord. al. alimentary 
 canal. H. Heart (bulbus arteriosus). Pp. Pleuroperitoneal cavity. 
 am. amnion. 
 
 On comparing this with Fig. 17, it will be seen that the mesoblast (muscular) 
 wall of the heart H has now become quite separate from the rest of the 
 mesoblast of the splanchnopleure, which forms, in the section, an independent 
 line below the heart, the section of branches of the omphalo-mesaraic veins 
 being seen on either side. The bridle of mesoblast represented in the drawing as 
 passing from the splanchnopleure below to the sounatopleure above, reaching the 
 latter just inside the fold of the anmion, has been described by His, but has 
 never been seen by ourselves. 
 
 In the vascular and pellucid areas, the formation of 
 vascular channels with a subsequent differentiation into 
 arteries, capillaries and veins, is proceeding rapidly. Blood- 
 
IV.] THE BLOOD-VESSELS. 69 
 
 corpuscles too are being formed in considerable numbers. 
 The mottled yellow vascular area becomes covered with red 
 patches consisting of aggregations of blood-corpuscles, often 
 spoken of as blood-islands. 
 
 Round the extreme margin of the vascular area and nearly 
 completely encircling it, is seen a thin red line, the sinus or 
 vena, terminal-is (Fig. 23, Sv.). This will soon increase in 
 size and importance. 
 
 From the vascular and pellucid area several large channels 
 are seen to unite and form two large trunks, one on either 
 side, which running along the splanchnopleure folds at nearly 
 right angles to the axis of the embryo, unite at the " point of 
 divergence" to join the venous end of the heart. These are 
 the omphalo-mesaraic veins (Figs. 14, o.f., 23, o.f.) spoken of 
 above. 
 
 Both vessels and corpuscles are formed entirely from the 
 cells of the mesoblast; and in the regions where the meso- 
 blast is cleft, are at first observed exclusively in the splanch- 
 nopleure. Ultimately of course they are found in the meso- 
 blast everywhere. 
 
 The mode of formation of the blood-vessels and corpuscles has been much 
 and long debated. The observations of one of us have led us to believe the 
 following to be the true account. 
 
 In the pellucid area, where the formation of blood-vessels may be most 
 easily observed, a number of mesoblastic cells are seen to send out processes. 
 These processes unite, and by their union a protoplasmic network is formed 
 containing nuclei at the points from which the processes started. The nuclei, 
 which as a rule are much elongated and contain large oval nucleoli, increase 
 very rapidly by division, and thus form groups of nuclei at the, so to speak, 
 nodal points of the network. Several nuclei may also be seen here and there 
 in the processes themselves. The network being completed, these groups, by con- 
 tinued division of the nuclei, increase rapidly in size; the majority of the nuclei 
 composing them acquire a red colour and become converted into blood-corpuscles 
 (Fig. 19, b.c.) ; but a few, generally on the outside of the group, remain un L 
 unaltered, (Fig. 19, a}. The protoplasm in which the central reddened nuclei 
 are imbedded becomes liquefied, while that on the outside of each group, as well 
 as that of the uniting processes, remains granular, and increasing in quantity, 
 forms an investment for the unaltered nuclei which are embedded in it. 
 
 Each nodal point is thus transformed into a more or less rounded mass of 
 blood-corpuscles floating in plasma but enveloped by a layer of nucleated proto- 
 plasm, the several groups being united by strands of nucleated protoplasm. These 
 uniting strands rapidly increase in thickness; new processes are also continually 
 being formed ; and thus the network is kept close and thickset while the area is 
 increasing in size. 
 
 JBy a transformation of nuclei similar to that which took place in the nodal 
 points, blood-corpuscles make their appearance in the processes also, the central 
 portions of which become at the same time liquefied. The uncoloured nuclei 
 
70 
 
 THE SECOND DAY. 
 
 [CHAP. 
 
 situate in the envelopes of the nodal groups, as well as those lying on the exterior 
 of the connecting processes, appropriate a quantity of the granular protoplasm 
 surrounding each, and thus become converted into spindle-shaped cells. Each 
 nodal group and each connecting process thus gets a distinct wall of nucleated 
 cells. By the continued widening of the connecting processes and solution of their 
 central portions, accompanied by a corresponding increase in the enveloping 
 nucleated cells, the original protoplasmic network is converted into a system 
 of communicating tubes, the canals of which contain blood-corpuscles and 
 plasma, and the walls of which are formed of spindle-shaped nucleated cells. 
 
 FIG. 19. 
 
 I.e.- 
 
 SURFACE VIEW FROM BELOW OF A SMALL PORTION OF THE POSTERIOR END OF 
 THE PELLUCID AREA OF A 36 HOURS' CHICK. To illustrate the formation of 
 the blood- capillaries and blood-corpuscles, magnified 400 diameters. 
 
 6. c. Blood- corpuscles at a nodal point, already beginning to acquire a red 
 colour. They are enclosed in masses of protoplasm in the outermost 
 layer of which are found nuclei, a, some of which contain two nucleoli. These 
 nuclei subsequently become the nuclei of the cells forming the walls of 
 the vessels. The nodal groups are united by protoplasmic processes (p.pr), 
 also containing nuclei with large nucleoli (ri). These nuclei increase in 
 number by division, and become converted in part into the nuclei of the 
 cells forming the walls of the vessels, and in part into blood-corpuscles. 
 
 The blood-corpuscles pass freely from the nodal points into the hollow pro- 
 cesses, and thus the network of protoplasm becomes a network of blood-vessels ; 
 the corpuscles and the nuclei of the walls of which have been by separate 
 paths of development derived from the nuclei of the original protoplasm. 
 
 The formation of the corpuscles does not proceed equally rapidly or to the 
 
JV.] THE BLOOD-VESSELS. 71 
 
 same extent in all parts of the blastoderm. By far the greater part are formed 
 in the vascular area, but some arise in the pellucid area, especially in the hinder 
 part. In the front of the pellucid area the processes are longer and the network 
 accordingly more open ; the corpuscles also are both later in appearing and 
 less numerous when formed. 
 
 The omphalo-mesaraic arteries and veins, and the sinus terminalis which 
 from the first has a distinct wall, seem to take origin in a manner altogether 
 similar to that of the smaller vessels ; and the description of the formation of 
 the heart which we gave above shews that it too is nothing but a gigantic 
 nodal point. 
 
 Assuming the truth of the above account, it is evident that the blood-vessels 
 of the chick do not arise as spaces or channels between adjacent cells of the 
 mesoblast, but are hollowed out in the communicating protoplasmic substance of 
 the cells themselves. It is also perhaps worthy of note that the red-blood 
 corpuscles are not cells, but nuclei. 
 
 The red- blood corpuscles when removed from the vessels exhibit energetic 
 amoeboid movements. They seem to increase at this stage chiefly by division. 
 
 The above is the view which we deduce from our own observations. The 
 following may serve as a brief summary of the history of the matter. 
 
 Von Baer and the older embryologists regarded the blood-vessels as being 
 at first mere gaps or spaces between the cellular elements of the mesoblast, 
 hollowed out so to speak by the now of blood from the heart. The first steps 
 in the right direction were taken by Remak and IQ>lliker, who described the 
 formation of solid bands or cylinders composed of cells and arranged in a 
 close-set network. These bands, becoming hollowed by solution while their 
 central cells were converted into blood-corpuscles, gradually put on the 
 appearance of blood-vessels, the aggregation of the red corpuscles at various 
 points, through arrest of the circulation, giving rise to the blood-islands of Wolfl 
 and Pander. 
 
 According to Af an assieff ( Wien. Sitz. Bericht. Bd. 53, 1866) there appear 
 in the mesoblast vesicles of variable size, with protoplasmic envelopes and 
 contents. These vesicles, which are at first clear and homogeneous, subse- 
 quently become traversed with strands of nucleated protoplasm, forming often a 
 close net-work within the vesicle. The space intervening between the nume- 
 rous vesicles is cut up into a network of canals by protoplasmic processes 
 stretching from one vesicle to another. These canals are the rudimentary 
 blood-vessels. From the outside of the vesicles, forming the inner wall of the 
 adjacent vessels, nucleated masses of protoplasm are budded off as blood-cor- 
 puscles and fall into the current of the circulation. 
 
 His (op. cit.), following out his peculiar theory of development, derived both 
 blood and blood-vessels from the white yolk or parablast. According to him 
 while certain of the white-yolk masses become converted into conglomerations 
 of cells, which acquiring a yellow colour stand out in suri'ace views as blood- 
 islands, other white-yolk masses, metamorphosed into angular cells, form a 
 network of thick lines permeating the mass of true blastodermic (archiblastic) 
 cells of the mesoblast. These lines, at the first solid, subsequently become 
 hollow. The meshwork of canals, or rudimentary blood-vessels, thus developed 
 first in the vascular and pellucid areas and spreading thence into the embryo, 
 contains for a certain time clear fluid only, the blood-islands being imbedded 
 in or attached to the walls of the canal and surrounded by protoplasmic 
 envelopes, so that the blood-corpuscles are shut out from the cavities of the 
 vessels. Later on, however, the envelopes of the islands are broken through, 
 and the blood -corpuscles emerging from their nests fall into the current of the 
 circulation. 
 
 His therefore regarded the blood- corpuscles as formed in greater part at least 
 
72 THE SECOND DAY. [CHAP. 
 
 separately from the blood-vessels ; their entrance into the vascular spaces being 
 an after event. The parablastic cells (derived from the white yolk), in his view, 
 give rise to the epithelium (endothelium) and connective tissue elements only 
 of the blood vessels, the muscular elements being derived from genuine (blasto- 
 dermic) mesoblastic cells. 
 
 Klein (Wien. Sitz. Bericht. LXIII. 1871) describes the blood-vessels as 
 taking their origin from certain cells of the mesoblast in which a vacuole, 
 appearing and rapidly increasing in size, pushes the nucleus on one side, 
 leaving only a thin layer of protoplasm round the periphery of the cell. In this 
 thin layer nuclei appear; and, multiplying, form a complete nucleated invest- 
 ment, to the vacuole, which meanwhile continues to increase in size. From the 
 inside of this protoplasmic investment cells are budded off, and fall into the 
 vacuole. Here they soon acquire a red colour and become converted into 
 blood-corpuscles. From the exterior of these vacuolated cells nucleated 
 processes are thrown out, which end freely or join with similar processes from 
 other cells. A protoplasmic network is thus formed, the lines of which become 
 vacuolated, and hollow, and ultimately communicate with the original central 
 vacuoles now crowded with corpuscles. By these means a system of com- 
 municating tubes is established. Klein also describes two other forms of 
 cells somewhat differing from the above, but also taking part in the formation 
 of the blood-vessels. One of these forms is found chiefly in the vascular area, 
 and he believes that these latter are simply the formative cells of which we 
 have already so often spoken. 
 
 It will thus be seen that Klein's view, from which our own differs chiefly in 
 reference to the matter of vacuolation, is a return, with some modifications and 
 extensions, to the earlier view of Rernak, and that the accounts of both Afanas- 
 sieff and His, which in turn agree in many respects, have proved to be uncorro- 
 borated divergences from the older track. 
 
 Still more recently Goette (Archiv fur Micro. Anat. Vol. X. 1873, pp. 
 145 199) has given an entirely different account of the origin of the blood- 
 vessels and bloo<l -corpuscles in the vascular area. He believes that in the 
 thick mass of cells immediately outside the 'pellucid area' (vide Chap. in. 12) 
 a quantity of fluid collects and causes the cells to separate into a network with 
 large spaces filled with fluid. Into these spaces the formative cells travel, and 
 undergoing a species of endogenous cell-formation, form masses of bl/od- 
 corpuscles the blood-islands of the ear.ier authors. This view differs, it will 
 be seen, from all the later views, and goes back to that of Von Baer in regard- 
 ing the blood-vessels as primitively mere gaps between the cellular elements. 
 In the investigation of such a point as this, sections (which apparently Goette 
 has alone employed) are very untrustworthy. 
 
 7. The cells of the epiblast and hypoblast as well as of the mesoblast 
 undergo considerable changes between the 24th and the 36th hour. 
 
 Up to the 24th hour the cells of both layers, but more especially of the 
 hypoblast, were filled with fine granules and also contained many highly 
 refractive spherules. By the 36th hour, however, they have become much 
 more transparent. Each cell now consists of a clear protoplasm with hardly 
 any granules or spherules, and a large oval nucleus together with one or more 
 vacuoles is distinctly visible. 
 
 The cells of the hypoblast still pass insensibly into the white-yolk cells ; 
 and it is still by the conversion of the white yolk into hypoblast that the 
 peripheral extension of the latter is chiefly carried on. 
 
 The hypoblast cells beneath and at the sides of the embryo are markedly 
 smaller than those at the periphery of the pellucid area. 
 
 The epiblast cells exhibit considerable variation in size in different parts of 
 
IV.] THE WOLFFIAN DUCT. 73 
 
 the embryo ; but all are considerably smaller and also somewhat more 
 columnar than the more peripheral cells of the pellucid area. The largest 
 epiblastic cells are to be found in the region of the vascular area, but here 
 they are much flattened. At the extreme outer edge of the opaque area the 
 cells are smaller again, shewing in this respect a marked contrast to their con- 
 dition during the previous stage. 
 
 8. About this period there may be seen in transverse 
 sections, taken through the embryo in the region of the proto- 
 vertebrae, a small group of cells (Fig. 20, W. d) projecting on 
 either side from the mass of uncleft mesobiast on the outside 
 of the protovertebrse, into the somewhat triangular space 
 formed by the epiblast above, the upper and outer angle of the 
 proto vertebra on the inside, and the mesobiast on the outside. 
 
 This group of cells is the section of a longitudinal ridge, 
 the rudiment of the Wolffian duct. We shall return to it 
 immediately. 
 
 9. The most important changes then which take place 
 during the first half of the second day, are the closure of the 
 medullary folds, especially in the anterior part, and the 
 dilatation of the canal so formed into the first cerebral 
 vesicle ; the establishment of a certain number of protoverte- 
 brse ; the elevation of the head from the plane of the 
 blastoderm ; the formation of the tubular heart and of the 
 great blood-vessels; and the appearance of the rudiment of 
 the Wolffian duct. It is important to remember that the 
 embryo of which we are now speaking is simply a part of the 
 whole germinal membrane, which is gradually spreading over 
 the surface of the yolk. It is important also to bear in mind 
 that all that part of the embryo which is in front of the most 
 anterior proto vertebrse corresponds to the future head, and 
 the rest to the neck, body and tail. At this period the head 
 occupies nearly a third of the whole length of the embryo. 
 
 10. The changes which take place from the 36th to 
 the 45th hour will best form the next stage, since those 
 which occur during the last few hours of the second day will 
 be more conveniently described with the third day. 
 
 One important feature of the stage is the rapid increase 
 in the process of the folding off of the embryo from the plane 
 of the germ, and its consequent conversion into a distinct 
 tubular cavity. At the beginning of the day, the head alone 
 projected from the rest of the germ, the remainder of the 
 
[CHAP. 
 
 TRANSVERSE SECTION THROUGH THE DORSAL REGION OF AN EMBRYO OF 
 
 45 HOURS. 
 
 A. epiblast. B. mesoblast. C. hypoblast consisting of a single row of flat- 
 tened cells. M. c. medullary canal. P. v. protovertebra. W. d. Wolffian 
 duct. S. o. Somatopleure. S. p. Splanchnopleure. p. p. pleuroperitoneal 
 
IV.] 
 
 THE TAIL-FOLD. 
 
 75 
 
 cavity, c. h. Dotochord. a. o. dorsal aorta, v. blood-vessels of the 
 yolk-sac, o. p. line of junction between opaque and pellucid areas ; 
 at this point the hypoblast cells are seen to pass without any strong 
 line of demarcation into the white-yolk spheres, w. white-yolk spheres, 
 some of which near the edge of the pellucid area contain a body very 
 like a nucleus. 
 
 Only one half of the section is represented in the figure if completed it 
 would be bilaterally symmetrical about the line of the medullary canal. 
 
 *. FIG. 21. 
 
 op.v? 
 
 EMBRYO OP THE CHICK AT 36 HOURS VIEWED FROM ABOVE AS AN OPAQUE 
 
 OBJECT. (Chromic acid preparation.) 
 /. b. front brain. ra. b. mid brain. h. b. hind brain, op. v. optic vesicle. 
 
 au. p. auditory vesicle, o. f. omphalo-mesaraic vein. p. v. protovertebra. 
 
 m. f. line of junction of the medullary folds above the medullary canal. 
 
 s. r. sinus rhomboidalis. t. tail-fold, p. r. remains of primitive groove. 
 
 a. p. area pellucida. 
 
 embryo being simply a part of a flat blastoderm, nearly 
 completely level from the front protovertebrse to the hind 
 edge of the pellucid area. At this epoch, however, a tail-fold 
 
76 THE SECOND DAY. [CHAP. 
 
 (Fig. 21, f) makes its appearance, elevating the tail above the 
 level of the blastoderm in the same way that the head was 
 elevated. Lateral folds also, one on either side, soon begin 
 to be very obvious. By the progress of these, together with the 
 rapid backward extension of the head-fold and the slower 
 forward extension of the tail-fold, the body of the embryo 
 becomes more and more distinctly raised up and marked 
 off from the rest of the blastoderm. 
 
 11. The medullary canal closes up rapidly. The wide 
 , sinus rhomboidalis becomes a narrow fusiform space (Fig. 21, 
 
 s.r.), and at the end of this period is entirely roofed over. The 
 conversion of the original medullary groove into a closed tube 
 is thus completed. 
 
 12. In the region of the head most important changes 
 now take place. We saw that at the beginning of this day 
 the front end of the medullary canal was dilated into a buib, 
 the first cerebral vesicle. This, from the very first broader 
 than long, now increases so much in breadth as to give the 
 embryo a hammer-headed appearance. The lateral portions, 
 continuing to enlarge, become after a while separated by 
 constrictions from the central portion. The single vesicle 
 is thus converted into three vesicles : a median one connected 
 by short hollow stalks with a lateral one on either side. The 
 lateral vesicles are known as the optic vesicles (Fig. 21, op. v, 
 Fig. 22, a), and will afterwards become converted into parts 
 
 HEAD OP A CHICK AT THE END OP THE SECOND DAT VIEWED FROM BELOW 
 AS A TRANSPARENT OBJECT. (Copied from Huxley). 
 
 /. first cerebral vesicle, a. optic vesicle, d. infundibulum. 
 
 The specimen shews the formation of the optic vesicles (a), as outgrowths 
 from the ist cerebral vesicle or vesicle of the 3rd ventricle, so that the optic 
 vesicles and vesicle of the 3rd ventricle at first freely communicated with each 
 other, and also the growth of the lower wall of the vesicle of the 3rd ventricle 
 into a process which becomes the infundibulum (d). 
 
IV.] THE OPTIC VESICLES. 77 
 
 of the eyes ; the median one still retains the name of the first 
 cerebral vesicle. The constriction takes place chiefly from 
 above downwards, so that the optic vesicles soon appear to 
 spring from the under portions of the cerebral vesicle. 
 
 The original vesicle being primarily an involution of the 
 epiblast, the walls of all three vesicles are formed of epiblast ; 
 all three vesicles are likewise covered over with the common 
 epiblastic investment which will eventually become the 
 epidermis of the skin of the head. Between this superficial 
 epiblast and the involuted epiblast of the vesicles, there 
 exists a certain quantity of mesoblast to serve as the material 
 out of which will be formed the dermis of the scalp, the 
 skull, and other parts of the head. At this epoch, however, 
 the mesoblast is found chiefly underneath the several vesicles. 
 A small quantity may in section be seen at the sides ; but 
 at the top the epidermic epiblast is either in close contact with 
 the involuted epiblast of the cerebral and optic vesicles or 
 separated from it by fluid alone, there being as yet in this 
 region no cellular elements between the two representing 
 the mesoblast. 
 
 The constrictions marking off the optic vesicles take 
 place of course beneath the cjmmon epiblastic investment, 
 which is not involved in them. As a consequence, though 
 easily seen in the transparent fresh embryo (Fig. 22), they 
 are but slightly indicated in hardened specimens (Fig. 21). 
 In sections they are very clearly seen. 
 
 13. When an embryo of the early part of the second day 
 is examined as a transparent object, that portion of the 
 medullary canal which lies immediately behind the first 
 cerebral vesicle is seen to be conical in shape, with its walls 
 thrown into a number of wrinkles. These wrinkles may 
 vary a good deal in appearance, and shift from time to time, 
 but eventually, before the close of the second day, after the 
 formation of the optical vesicles, settle down into two con- 
 strictions, one separating the first cerebral vesicle from that 
 part of the medullary canal which is immediately behind 
 it, and the other separating that second portion from a third. 
 So, instead of there being one cerebral vesicle only, as at the 
 commencement of the second day, there is now, in addition 
 to the optic vesicles, a series of three, one behind the other ; 
 a second and third cerebral vesicle have been added to the 
 
78 THE SECOND DAY. [CHAP. 
 
 first (Fig. 21, ml, hb). They may be also called the "fore 
 brain," the " mid brain," and the " hind brain," for into these 
 parts will they eventually be developed. 
 
 14. The optic vesicles, lying underneath the epiblast, 
 towards the end of the day are turned back and pressed 
 somewhat backwards and downwards against the sides of 
 the first cerebral vesicle or fore brain, an elongation of 
 their stalks permitting this movement to take place. The 
 whole head becomes in consequence somewhat thicker and 
 rounder. 
 
 15. Before the end of the day the fore brain, by a pro- 
 cess similar to that whereby the optic vesicles were formed, 
 viz. undue growth followed by constriction, has begun to 
 bud off two small vesicles in front ; these are the vesicles 
 of the cerebral hemispheres, which subsequently become the 
 most conspicuous part of the brain, but up to the end of the 
 day are still very small and inconspicuous. 
 
 16. The notochord, whose origin was described in the 
 account of the first day (Chap. III. 5), is during the whole of 
 the second day a very conspicuous object. It is seen as a trans- 
 parent rod, somewhat elliptical in section (see Fig. 20, ch} 9 
 lying immediately underneath the medullary canal for the 
 greater part of its length, and reaching forward in front as 
 far as below the centre of the second cerebral vesicle, where 
 it ends either in a point (Remak), or in a rounded knob 
 (Baer, Dursy, Entwickelungsgeschichte des Kopfes). The ex- 
 act relations of its termination will be discussed later on. 
 
 Round the anterior termination of the notochord, the 
 medullary canal, which up to the present time has remained 
 perfectly straight, towards the end of the day begins to curve. 
 The front portion of the canal, i*e. the fore-brain with its 
 optic and cerebral vesicles, becomes slightly bent downwards, 
 so as to form a rounded obtuse angle with the rest of the 
 embryo. This is the commencement of the so-called cranial 
 flexure. 
 
 17. Lastly, as far as the head is concerned, the rudiment 
 of the ear appears about this time on the dorsal surface as 
 a small depression or pitting of the epiblast on either side of 
 the hind -brain (Fig. 21, au. p). 
 
 18. We left the heart as a fusiform body slightly bent to 
 the right, attached to the under wall of the foregut by the 
 
IV.] THE CIRCULATION. 79 
 
 aorta and by its venous end, but with its intermediate portion 
 quite free. The curvature now increases so much that the 
 heart becomes almost m -shaped, the venous portion being 
 drawn up towards the head so as to lie somewhat above (dorsal 
 to) and behind the arterial portion. (It would perhaps be more 
 correct to pay that the free intermediate portion is by its 
 own growth bent downwards, backwards, and somewhat to 
 the right, while the venous root of the heart is at the same 
 time continually being lengthened by the carrying back of 
 that " point of divergence " of the splanchnopleure folds which 
 marks the union of the omphalo-mesaraic veins into a single 
 venous trunk). The heart then has at this time two bends, 
 the one, the venous bend, the right-hand curve of the CQ ; 
 the other, the arterial bend, the left-hand curve of the ^ . 
 The venous bend which, as we have said, is placed alsove 
 arid somewhat behind the arterial bend, becomes marked by 
 two bulgings, one on either side. These are the rudiments 
 of the auricles, or rather of the auricular appendages. The 
 ascending limb of the arterial bend soon becomes conspicuous 
 as the bulbus arteriosus, while the rounded point of the 
 bend itself will hereafter grow into the ventricles. 
 
 19. The blood-vessels, whose origin during the first half 
 of this day has been already described, become during the 
 latter part of the day so connected as to form a complete 
 system, through which a definite circulation of the blood is now 
 for the first time (consequently some little while after the 
 commencement of the heart's pulsation) carried on. 
 
 The two primitive aortce have already been described as 
 encircling the foregut, and then passing along the body of 
 the embryo immediately beneath the protovertebrse on either 
 side of the notochord. They are shewn in Fig. 20 a.o in section 
 as two large rounded spaces lined with spindle-shaped cells. 
 At first they run as- two distinct canals along the whole 
 length of the embryo ; but, after a short time, unite at some 
 little distance behind the head into a single trunk, which 
 lies in the middle line of the body immediately below the 
 notochord (Fig. 39). Lower down, nearer the tail, this 
 single primitive trunk again divides into two aortse, which, 
 getting smaller and smaller, are finally lost in the small 
 blood-vessels of the tail. At this epoch, therefore, there are 
 two aortic arches springing from the bulbus arteriosus, and 
 
80 THE SECOND DAY. [CHAP. 
 
 uniting above the alimentary canal in the back of the 
 to form the single dorsal aorta, which travelling backwards in 
 the median line divides near the tail into two main branches. 
 From each of the two primitive aortae, or from each of the 
 two branches into which the single aorta divides, there is 
 given off on either side a large branch. These have been 
 already spoken of as the omphalo-mesaraic arteries. At 
 this stage they are so large that by far the greater part of 
 the blood passing down the aorta finds its way into them, 
 and a small remnant only pursues a straight course into the 
 continuations of the. aorta towards the tail 
 
 Each omphalo-mesaraic artery leaving the aorta at nearly 
 right angles (at a point some little way behind the backward 
 limit of the splanchnopleure fold which is forming the ali- 
 mentary canal), runs outwards beneath the protovertebrse in 
 the lower range of the mesoblast, close to the hypoblast. 
 Consequently, when in its course outwards it reaches the 
 point where the mesoblast is cleft to form the somatopleure 
 and splanchnopleure, it attaches itself to the latter. Travel- 
 ling along this, and dividing rapidly into branches, it reaches 
 the vascular area in whose network of small vessels (and also 
 to a certain extent in the similar small vessels of the pellucid 
 area) it finally loses itself. 
 
 The terminations of the omphalo-mesaraic arteries in the 
 vascular and pellucid areas are further connected with the 
 heart in two different ways. From the network of capillaries, 
 as we may call them, a number of veins take their origin, 
 and finally unite into two main trunks, the omphalo-mesaraic 
 veins. These have already been described as running along the 
 folds of the splanchnopleure to form the venous roots of the 
 heart. Their course is consequently more or less parallel to 
 that of the omphalo-mesaraic arteries, but at some little dis- 
 tance nearer the head, inasmuch as the arteries run in that 
 part of the splanchnopleure which has not yet been folded in 
 to form the alimentary canal. Besides forming the direct 
 roots of the omphalo-mesaraic veins, the terminations of 
 the omphalo-mesaraic arteries in the vascular area are also 
 connected with the sinus terminalis spoken of above as run- 
 ning almost completely round, and forming the outer margin 
 of the vascular area. This (Fig. 23, s.v), may be best de- 
 scribed as composed of two semicircular canals, which nearly 
 
IV.] THE AORTIC ARCHES. 81 
 
 meet at points opposite the head and opposite the tail, 
 thus all but encircling the vascular area between them. 
 At the point opposite the head the end of each semi- 
 circle is connected with vessels (Fig. 23), which run straight 
 in towards the heart along the fold of the splanchnopleure, 
 and join the right and left omphalo-mesaraic veins. At 
 the point opposite the tail there is at this stage no such 
 definite connection. At the two sides, midway between 
 their head and tail ends, the two semicircles are espe- 
 cially connected with the omphalo-mesaraic arteries. 
 
 The circulation of the blood then during the latter half 
 of the second day may be described as follows. The blood 
 brought by the omphalo-mesaraic veins falls into the twisted 
 cavity of the heart, and is driven thence through the bulbus 
 arteriosus and aortic arches into the aortic trunk. From the 
 aorta, by far the greater part of the blood flows into the 
 omphalo-mesaraic arteries, only a small remnant passing on 
 into the caudal terminations. From the capillary net-work 
 of the vascular and pellucid area into which the omphalo- 
 mesaraic arteries discharge their contents, part of the blood 
 is gathered up at once into the lateral or direct trunks 
 of the omphalo-mesaraic veins. Part however goes into 
 the middle region of each lateral half of the sinus termi- 
 nalis, and there divides on each side into two streams. One 
 stream, and that the larger one, flows in a forward direction 
 until it reaches the point opposite the -head, thence it 
 returns by the veins spoken of above, straight to the 
 omphalo-mesaraic trunks. The other stream flows backward, 
 and becomes lost at the point opposite to the tail. This is 
 the condition of things during the second day ; it becomes 
 considerably changed on the succeeding day. 
 
 At the time that the heart first begins to beat the 
 capillary system of the vascular and pellucid areas is not yet 
 completed; and the fluid which is at first driven by the heart 
 contains, according to most observers, very few corpuscles. 
 
 20. At the close of the second day the single, pair of 
 aortic arches into which the bulbus arteriosus divides is 
 found to be accompanied by a second pair, formed in the 
 same way as the first, and occupying a position a little 
 behind it. Sometimes even a third pair is added. Of these 
 aortic arches we shall have to speak more fully later on. 
 E. 6 
 
82 THE SECOND DAY\ [CHAP. 
 
 21. At the latter end of this day, the ridge which we 
 have already spoken of as the rudiment of the Wolffian duct, 
 has become distinctly hollow, is in fact no longer a ridge but 
 a canal. Sections now shew not an irregular group of ordi- 
 nary mesoblastic cells, but a small cavity surrounded by a 
 wall of colls ; and these cells are beginning to put on a 
 columnar character, and thus appear to radiate from the cen- 
 tral cavity. The canal or duct so formed, the anterior 
 termination of which is closed, and the posterior not as 
 yet completely formed, reaches from about the fifth pair 
 of protovertebrse backwards towards the hind end of the 
 embryo. The conversion of the ridge into a canal appears 
 to take place by the cells acquiring a radiating arrange- 
 ment, and a small hole appearing at the centre where the 
 points of the cells meet ; this rapidly grows larger till it 
 reaches the final size of the cavity of the duct, 
 
 The exact mode of development of the Wolffian duct is still a matter of some 
 doubt, although its origin has been investigated by numerous embryologists. 
 
 Remak, and after him Kolliker, described it as taking its origin from the 
 mesoblast of the somatopleure, and appearing about the middle of the second 
 day, at the external border of the protovertebrae immediately under the epi- 
 blast, in the form of a solid cord which subsequently became hollow. 
 
 liursy (Zeitsch. f. Rat. Med. 1865) gave a very similar account, except that 
 he regarded it as being derived from the substance of the proto vertebrae, 
 instead from the somatopleure. 
 
 Hensen (Archiv. Microscop. Anat. Bd. in. 1867), an ^ f r 8ome time His, 
 believed that the duct took origin as a longitudinal involution of the epillast 
 between the protovertebrse and the lateral mesoblast, in the form of a groove, 
 which subsequently became closed in and detached from the superficial epiblast, 
 in a manner very similar to the way in which the lens is formed. 
 
 Subsequently His took up the view that it was a product of the proto- 
 vertebrae, the central cells of these bodies, according to him, protruding as a 
 ridge along their upper and external angles. He states that at first a distinct 
 connection ia visible between the Wolffian duct and the central cells of the 
 protovertebrse. 
 
 Waldeyer (Eierstock u. Ei, 1870) has given a totally different account. 
 Between the external border of the protovertebres, and the point where the 
 mesoblast splits into somatopleure and splaiichnopleure, there lies a mass of 
 cells, which we shall have occasion to speak of hereafter as the intermediate cell- 
 mass. According to Waldeyer, the upper surface of this mass grows up into 
 a narrow ridge, seen in sections as a tongue-shaped process projecting into the 
 vacant space (i. e. the space filled with fluid only) which exists below the 
 epiblast at this point, Later on, this tongue-shaped process is seen to curve out- 
 wards, and thus to become hook-shaped ; and the point of the hook subsequently 
 unites with a similar smaller process derived from the more external portions 
 of the same cell-mass. The small cavity thus seen to be enclosed by a larger 
 and smaller process, is of course the sectional view of a canal enclosed by a 
 larger and smaller lidge. This canal is the Wolffian duct. Waldeyer further 
 believes that the cells which thus form the walls of the duct are primarily epi- 
 
IV.] THE AMNION. 83 
 
 blastic in origin, having been separated from the epiblast at the epoch of the 
 apparent fusion of the epiblast and mesoblast in the region of the primitive 
 streak or axis-cord of His This view, prompted as it evidently is by theoretical 
 considerations, must be regarded as untenable, since the primitive streak has 
 nothing whatever to do with the permanent body of the embryo. 
 
 Quite recently Romiti (Archiv. Microscop. Anat. x. 1874) has described the 
 Wolffian duct as being formed by an involution of the epithelium of die pleuro- 
 peritoneal cavity, in the form of a longitudinal groove which is thrust up into 
 the superior portions of the intermediate cell-mass, and the communication of 
 which with the pleuroperitoneal cavity is speedily obliterated. Such a mode 
 of origin recommends itself to the embryologtetj inasmuch as it is certainly the 
 way in which, as we shall see, the Wolffian duct is formed in Amphibia and 
 Osseous Fishes. For that very reason however it should be received with 
 caution; all the more since the sections drawn by Romiti, and described as 
 supporting his views, evidently belong to a stage considerably later than that 
 at which the duct first distinctly appears. We hope to be able to shew, in the 
 second part of this work, that the mode of development of the Wolffian duct 
 described above, and which we believe to be the real one> is not so abnormal as 
 it might at first sight be supposed to be. 
 
 22. The amnion, especially the anterior or head-fold, 
 advances in growth very rapidly during the second day, and 
 at its close completely covers the head and neck of the 
 embryo ; so much so that it is necessary to tear or remove it 
 when the head has to be examined in hardened opaque speci- 
 mens. The tail and lateral folds of the amnion, though still 
 progressing, lag considerably behind the head-fold. 
 
 23. The chief events then which occur during the 
 second half of the second day are as follows ; 
 
 1. The second and third cerebral vesicles make their 
 appearance behind the first. 
 
 2. The optic vesicles spring as hollow buds from the 
 lateral, and the vesicles of the cerebral hemispheres from the 
 front portions of the first cerebral vesicle. 
 
 3. The first rudiment of the ear is formed as an involu- 
 tion of the epiblast on the side of the hind-brain or third 
 cerebral vesicle. 
 
 4. The first indications of the cranial flexure become 
 visible. 
 
 5. The head-fold, and especially the splanchnopleure 
 moiety, advances rapidly backwards ; the head of the embryo 
 is in consequence more definitely formed, The tail-fold also 
 becomes distinct. 
 
 6. The curvature of the heart increases ; the first rudi- 
 ments of the auricles appear. 
 
 7. The circulation of the yolk-sac is completed. 
 
 8. The amnion grows rapidly. 
 
CHAPTER V. 
 
 THE CHANGES WHICH TAKE PLACE DURING THE THIRD DAY. 
 
 1. OF all days in the history of the chick within the egg 
 this perhaps is the most eventful ; the rudiments of so many 
 important organs now first make their appearance. 
 
 On opening an egg on the third day, the first thing which 
 attracts notice is the diminution of the white of the egg. 
 This seems to be one of the consequences of the functional 
 activity of the newly-established vascular area whose blood- 
 vessels are engaged either in directly absorbing the white or, 
 as is more probable, in absorbing the yolk, which is in turn 
 replenished at the expense of the white. The absorption, 
 once begun, goes on so actively that, by the end of the day, 
 the decrease of the white is very striking. 
 
 2. The blastoderm has now spread over about half the 
 yolk, the extreme margin of the opaque area reaching about 
 half-way towards the pole of the yolk opposite to the embryo. 
 
 The vascular area, though still increasing, is much smaller 
 than the total opaque area, being in average-sized eggs about 
 as large as a florin. Still smaller than the vascular area is the 
 pellucid area in the centre of which lies the rapidly growing 
 embryo. 
 
 3. During the third day the vascular area is not only a 
 means for providing the embryo with nourishment from the 
 yolk, but also, inasmuch as by the diminution of the white it 
 is brought close under the shell and therefore fully exposed 
 to the influence of the atmosphere, serves as the chief organ 
 of respiration. 
 
 This in fact is the period at which the vascular area may 
 be said to be in the stage of its most complete development ; 
 
CH. V.] 
 
 THE VASCULAR AREA. 
 
 85 
 
 for though it will afterwards become larger it will at the 
 same time become less definite and less important. We 
 may therefore, before we proceed, add a few words to the 
 description of it given in the last chapter. 
 
 L.Of 
 
 DIAGRAM OP THE CIRCULATION OF THE YOLK-SACK AT THE END OF THE 
 THIRD DAY OF INCUBATION. 
 
 H. heart. A A. the second, third and fourth aortic arches; the first 
 has become obliterated in its median portion, but is continued at its 
 proximal end as the external carotid, and at its distal end as the internal 
 carotid. AO. dorsal aorta. L. Of. A. left omphalo-mesaraic artery. 
 R. Of. A. right omphalo-mesaraic artery. S. T. sinus terminals. L. Of. left 
 omphalo-mesaraio vein. R. Of. right omphalo-mesaraic vein. S. V. sinus 
 venosus. D. C. ductus Cuvieri. S. Ca. V. superior cardinal vein. 
 V.Ca. inferior cardinal vein. The veins are marked in outline and the arteries 
 are made black. The whole blastoderm has been removed from the egg 
 and is supposed to be viewed from below. Hence the left is seen on the 
 right, and vice versa. 
 
86 THE THIKD DAY. [CHAP. 
 
 The blood leaving the body of the embryo by the omphalo- 
 mesaraic arteries (Fig. 23, R. Of.A., L. Of. A.), is carried to 
 the small vessels and capillaries of the vascular area, a small 
 portion only being appropriated by the pellucid area. 
 
 From the vascular area part of the blood returns directly 
 to the heart by the main lateral trunks of the omphalo- 
 mesaraic veins, R. Of., L. Of. During the second day these 
 venous trunks joined the body of the embryo considerably 
 in front of, that is, nearer the head than, the corresponding 
 arterial ones. Towards the end of the third day, owing to the 
 continued lengthening of the heart, the veins and arteries 
 run not only parallel to each other, but almost in the same 
 line, the points at which they respectively join and leave 
 the body being nearly at the same distance from the head. 
 According to Von Baer and other observers the veins in the 
 vascular area are placed nearer the surface thau are the 
 arteries. Close to the body the reverse is the case ; near the 
 body therefore they cross over each other. 
 
 The rest of the blood brought by the omphalo-mesaraic 
 arteries finds its way into the lateral portions of the sinus 
 terminalis, S.T., and there divides on each side into two 
 streams. Of these, the two which, one on either side, flow 
 backward, meet at a point about opposite to the tail of the 
 embryo, and are conveyed along a distinct vein which, run- 
 ning straight forward parallel to the axis of the embryo, 
 empties itself into the left omphalo-mesaraic vein. The 
 two forward streams reaching the gap in the front part of 
 the sinus terminalis fall into either one, or in some cases 
 two veins, which run straight backward parallel to the axis 
 of the embryo, and so reach the roots of the heart. When 
 one such vein only is present it joins the left omphalo- 
 mesaraic trunk ; where there are two they join the left and 
 right omphalo-mesaraic trunks respectively. The left vein is 
 always considerably larger than the right; and the latter 
 when present rapidly gets smaller and speedily disappears. 
 
 The chief differences then between the peripheral cir- 
 culation of the second and of the third day are due to the 
 greater prominence of the sinus terminalis and the more 
 complete arrangements for returning the blood from it to the 
 heart. After this day, although the vascular area will go on 
 increasing in size until it finally all but encompasses the 
 
V.] CHANGE OF POSITION. 87 
 
 yolk, the prominence of the sinus terminalis will become less 
 and less in proportion as the respiratory work of the vascular 
 area is shifted on to the allantois, and its activities confined 
 to absorbing nutritive matter from the yolk. 
 
 4. The folding in of the embryo makes great progress 
 during this day. Both head and tail have become most 
 distinct, and the side folds which are to constitute the 
 lateral walls have advanced so rapidly that the embryo is 
 now a bona fide tubular sac, connected with the rest of 
 the yolk by a broad stalk. This stalk, as was explained in 
 Chap, n, is double, and consists of an inner splanchnic stalk 
 continuous with the alimentary canal, which is now a tube 
 closed at both ends and open to the stalk along its middle 
 third only, and an outer somatic stalk continuous with the 
 body- walls of the embryo, which have not closed nearly to 
 the same extent as the walls of the alimentary canal. (Com- 
 pare Fig. 8. A and B, which may be taken as diagrammatic 
 representations of longitudinal and transverse sections of an 
 embryo of this period.) 
 
 5. The embryo is almost completely covered by the 
 amnion. Before the close of the day the several amniotic 
 folds will have met along a line over the back of the embryo. 
 Their complete coalescence, and the obliteration of their line 
 of junction, will however not take place till the following 
 day. 
 
 6. During this day a most rema.rkable change takes 
 place in the position of the embryo. Up to this time it has 
 been lying symmetrically upon the yolk with the part 
 which will be its mouth directed straight downwards. It 
 now turns round so as to lie on its left side. 
 
 This important change of position is almost invariably 
 completed on the third day. At the same time the left 
 omphalo-mesaraic vein, the one on the side on which the 
 embryo comes to lie, grows very much larger than the 
 right, which henceforward gradually dwindles and finally dis- 
 appears. 
 
 7. Coincidently with the change of position the whole 
 embryo begins to be curved on itself. This curvature of the 
 body, Fig. 46, becomes still more marked on the fourth day. 
 
 8. In the head very important changes take place. 
 One of these is the cranial flexure. Figs. 24, 25. This (which 
 
THE THIRD DAY. 
 
 [CHAP; 
 
 must not be confounded with the curvature of the body just 
 referred to) we have already seen was commenced in the 
 course of the second day, by the bending downwards of the 
 head round a point which may be considered as the extreme 
 end either of the notochord or of the alimentary canal. 
 
 CHICK OF THE THIRD DAY (54 HOURS) VIEWED FROM UNDERNEATH AS A 
 TRANSPARENT OBJECT. 
 
 a', the outer amniotic fold or false amnion. This is very conspicuous around 
 
 the head, hut may also be seen at the tail. 
 a. the true anmion, very closely enveloping the head, and here seen only between 
 
 the projections of the several cerebral vesicles. It may also be traced at 
 
 the tail, t. 
 
 In the embryo of which this is a drawing the head-fold of the amnion 
 reached a little farther backward than the reference u, but its limit could not be 
 
V.] THE CRANIAL FLEXURE. 89 
 
 distinctly seen through the body of the embryo. The prominence of the false 
 atnnion at the head is apt to puzzle the student ; but if he bears in mind the 
 fact, which could not well be shewn in Fig. 8, that the whole arnniotic fold, both 
 the true and the false limb, is tucked in underneath the head, the matter will 
 on reflection become intelligible. 
 
 C. H. cerebral hemisphere. F. B. fore-brain or vesicle of the third ventricle. 
 M '. B. mid-brain. H. B. hind-brain. Op. optic vesicle. Ot. otic vesicle. 
 
 OfV. omphalo-mesaraic veins forming the venous roots of the heart. The trunk 
 on the right hand (left trunk when the embryo is viewed in its natural 
 position from above) receives a large branch, shewn by dotted lines, coming 
 from the anterior portion of the sinus terminalis Ht. the heart, now com- 
 pletely twisted on itself. Ao. the bulbus arteriosus, the three aortic arches 
 being dimly seen stretching from it across the throat, and uniting into the 
 aorta, sti.l more dimly seen as a curved dark line running along the body. 
 The other curved dark line by its side, ending near the reference y, is the 
 notochord ch. 
 
 About opposite the line of reference x the aorta divides into two trunks, which 
 running in the line of the somewhat opaque proto vertebrae on either side, 
 are not clearly seen. Their branches however, Ofa t the omphalo-mes'araic 
 arteries, are conspicuous and are seen to curve round the commencing side 
 folds. 
 
 Pv. protovertebrae. Below the level of the omphalo-mesaraic arteries the vertebral 
 plates are but imperfectly cut up into proto vertebrae, and lower down still, 
 not at all. 
 
 x is placed at the "point of divergence" of the splanchnopleure folds. The 
 blind foregut begins here and extends about up to y, the more transparent 
 space marked by that letter being partly due to the presence there of the 
 cavity of the alimentary canal, x therefore marks the present hind limit of 
 the splanchnopleure folds. The limit of the more transparent somato- 
 pleure folds cannot be seen. 
 It will be of course understood that all the body of the embryo above the 
 
 level of the reference x, is seen through the portion of the yolk-sac (vascular 
 
 and pellucid area), which has been removed with the embryo from the egg, as 
 
 well as through the double amniotic fold. 
 
 We may repeat that, the view being from below, whatever is described in 
 
 the natural position as being to the right here appears to be left, and vice 
 
 versa. 
 
 The flexure progresses rapidly, the front-brain being 
 more and more folded down till, at the end of the third day, 
 it is -no longer the first vesicle or fore-brain, "but the second 
 cerebral vesicle or mid-brain, which occupies the extreme 
 front of the long axis of the embryo. In fact a straight 
 line through the long axis of the embryo would now pass 
 through the mid-brain instead of, as at the beginning of the 
 second day, through the fore-brain, so completely has the 
 front end of the neural canal been folded over the end of 
 the notochord. The commencement of this cranial flexure 
 gives the body of an embryo of the third day somewhat the 
 appearance of a retort, the head of the embryo corresponding 
 
00 
 
 THE THIRD DAY. 
 
 [CHAP. 
 
 to the bulb. On the fourth day the flexure is still greater 
 than on the third, but on the fifth and succeeding days it 
 becomes less obvious owing to the filling up of the parts of the 
 skull. 
 
 FIG. -25. 
 
 HEAD OP A CHICK OF THE THIRD DAY VIEWED SIDEWAYS AS A TRANSPARENT 
 OBJECT. (From Huxley.) 
 
 I. a. the vesicle of the cerebral hemisphere. I. 6. the vesicle of the third 
 ventricle (the original fore-brain) ; at its summit is seen the projection of 
 the pineal gland e. 
 
 Below this portion of the brain is seen, in optical section, the optic vesicle ct 
 already involuted with its thick inner and thinner outer wall (the letter a is 
 placed on the junction of the two, the primary cavity being almost obliterated). 
 In the centre of the vesicle lies the lens, the shaded portion (represented too 
 large), being the expression of its cavity. Below the lens between the two limbs 
 of the horse-shoe is the choroidal fissure. 
 
 II. the mid-brain, now, owing to the cranial flexure, opposite the end of 
 the alimentary canal. III. the hind-brain. V. the rudiments of the fifth 
 cranial nerve, VII. of the seventh. Below the seventh nerve is seen the 
 auditory vesicle b. The head having been subjected to pressure, the vesicle 
 appears somewhat distorted as if squeezed out of place. The orifice is not yet 
 quite closed up. 
 
 i. the inferior maxillary process of the first visceral fold. Below, and to 
 the right of this, is seen the first visceral cleft, below that again the second 
 visceral fold (2), and lower down the third (3) and fourth (4) visceral folds. In 
 front of the folds (i.e. to the left) is seen the arterial end of the heart, the 
 aortic arches being buried in their respective visceral folds. 
 
 f. represents at its lowest part the cavity of the alimentary canal as seen 
 through the transparent body of the embryo; at the upper part below the brain 
 it is difficult to distinguish between the transparency due to the presence of the 
 cavity of the alimentary canal, and that caused by the character of the meso- 
 blast at the base of the skull, which, being formed of stellate cells with largely 
 developed clear spaces or vacuoles, allows the light to pass readily through it. 
 Near its upper end below the mid-brain is seen a small conical process, the 
 rudiment of the infundibulum. 
 
 9. The two vesicles of the cerebral hemispheres which 
 
V.] THE FORE-BRAIX. 91 
 
 on the second day began to grow out from the front of the 
 fore-brain, increase rapidly in size during the third day, so 
 much so that by the end of the day each of them (Fig. 24, C7/, 
 Fig. 25, la) is as large or larger than the original fore-brain 
 from which they both sprang, and they form together a most 
 conspicuous part of the brain. In their growth they pash 
 aside, the optic vesicles, and thus contribute largely to the 
 roundness which the head is now acquiring. Each vesicle 
 possesses a cavity, known afterwards as a lateral ventricle , 
 which, though quite separate from its fellow, is continuous 
 with the cavity of the fore-brain. 
 
 Owing to the development of these cerebral hemispheres, 
 the original fore-brain no longer occupies the front position 
 (Fig. 24, FB, Fig. 25, 16), and ceases to be the conspicuous 
 object that it was. Inasmuch as its walls will hereafter be de- 
 veloped into the parts surrounding the so-called third ventricle 
 of the brain, we shall henceforward speak of it as the vesicle 
 of the third ventricle, or, inasmuch as it soon comes to lie 
 between the expanded posterior ends of the cerebral hemi- 
 spheres, as the 'tween brain. 
 
 On the summit of the 'tween brain there may now be 
 seen a small conical projection, the rudiment of the pineal 
 gland' (Fig. 25, e\ while the centre of the floor is produced 
 into a funnel-shaped process, the infundibulum (Fig. 22, d\ 
 which, stretching towards the extreme end of the alimentary 
 canal, joins the pituitary body. 
 
 The development of the pituitary body or hypophysis cerebri has been the 
 subject of considerable controversy amongst embryologists. Von Baer (loc. cit.) 
 and Smidt (Zeitschrift fur Wiss. Zoologie, 1862, JB. XT, p. 43) believed that the 
 base of the fore-brain, or vesicle of the third ventricle, became produced into a 
 downward process, the 'infundibulum,' which subsequently became expanded at 
 its termination to form the pituitary body. 
 
 Kathke (Archiv fiir Anatomic und Physiologic 1838, Bd. v.) states that very 
 early a diverticulum is produced from the upper end of the alimentary canal, which 
 passes backwards and meets the process of the brain called the infundibulum. 
 This diverticulum subsequently loses all connection with the epithelium of the 
 : digestive canal, and, uniting with the infundibulum, forms the pituitary body. 
 
 Dursy (Entwicklungsgeschichte des Kopfes, Tubingen, 1869) states that both 
 ithe end of the notochord and the epithelium of the alimentary canal take, part 
 j in the formation of the pituitary body. The apparent diverticulum of the ali- 
 1 mentary canal is not so much a true diverticulum, as a part of the alimentary 
 , can;il constricted off from the remainder by the cranial flexure. 
 
 Keichert (Entivicklungsleben im Wirbelthierreich. Berlin, 1840) states that 
 the pituitary body is formed from the remains of the front end of notochord. 
 
92 THE THIRD DAY. [CHAP. 
 
 Subsequently however (Der Ban des menschlichen Gehirns) he supposed that 
 it was formed from the pia mater. 
 
 Rathke also subsequently (EntwicTclunsgescJiiclite der Wirbelthiere, Leipzig, 
 1861) gave up his former view, and believed that the diverticulum of the 
 alimentary canal disappeared, but that the pituitary body was formed from 
 the mesoblast in front of the clinoid process. 
 
 Wilhelm Miiller (Ueber die Entwicklung und Ban der Hypophysis und des 
 Processes infundibuli cerebri. Jenaiscke Zeitschrift, Bd. vr. 1871) has recently 
 written an elaborate memoir on the development and anatomy of the pituitary 
 body and infundibulum in all the orders of Vertebrates, of which the following 
 is an abstract. 
 
 In order to understand the formation of the diverticulum from the ali- 
 mentary canal which forms the pituitary body, we must remember that at 
 first the hypoblast of the throat closely underlies the notochord, and beyond 
 the end of the notochord is almost in contact with the base of the vesicle of the 
 third ventricle. When the cranial flexure occurs, which it will be remembered 
 takes place about an axis coinciding with the end of the notochord, the 
 hypoblast, which closely underlies the base of the brain, becomes at the same 
 time bent; and as the angle of the flexure becomes an acute angle, a wedge- 
 shaped space lined by hypoblast is as it were constricted off from the alimen- 
 tary canal. In this way there is formed a diverticulum of hypoblast which 
 passes forwards from the alimentary canal to the base of the fore-brain 
 a diverticulum not produced by a forward growth from the alimentary canal, 
 but solely due to the cranial flexure constricting off a wedge-shaped portion 
 of the alimentary canal. This we may call the pituitary diverdculum. When 
 the cranial flexure commences the end of the notochord becomes bent down- 
 ward, and, ending in a somewhat enlarged extremity, comes in contact with the 
 termination of the pituitary diverticulum. The mesoblast around and at the 
 front of the end of the notochord increases and grows up, in front of the 
 notochord and behind the vesicle of the third ventricle, to form the posterior 
 clinoid process. The base of the vesicle of the third ventricle at the same 
 time grows downwards towards the pituitary diverticulum and forms what is 
 known as the infundibulum. This state of things may be observed on the 
 third day. On the fourth day the mesoblast tissue around the notochord 
 increases in quantity, and the end of the notochord, though still bent down- 
 wards, recedes a little from the termination of the pituitary diverticulum, which 
 is still a triangular space with a wide opening into the alimentary canal. 
 
 On the fifth day, the opening of the pituitary diverticulum into the 
 alimentary canal lias become narrowed* and around the whole diverticulum a 
 formation of mesoblast-cells lias commenced. Behind it the clinoid process 
 has become cartilaginous, while to the side and in front it is enclosed by the 
 trabeculse. At this stage, in fact, we have a diverticulum from the alimentary 
 canal passing through the base of skull to the infundibulum. The end of the 
 notochord has at this stage become atrophied, so that it is separated by a 
 considerable interval from the pituitary body. 
 
 On the seventh day the mesoblast around the pituitary diverticulum has 
 grown into a complete investment of spindle-shnped cells, and the communi- 
 cation between the cavity of the diverticulum and that of the throat has become 
 still narrower. The diverticulum is all but converted into a vesicle, and its hypo- 
 blast walls have commenced to send out into the mesoblastic investment solid 
 processes, which form the first commencement of the true pituitary body. The 
 infundibulum now appears as a narrow- process from the base of the vesicle of 
 the third ventricle, which approaches, but does not unite with the pituitary vesicle. 
 This latter lies in the space between the basi- and the presphenoid, and is 
 
V.] THE MID-BRAIN AND HIND-BRAIN. 93 
 
 completely surrounded by a ring of cartilage. The mesoblast-cells immediately 
 around it do not. however, exhibit any signs of becoming cartilaginous. 
 
 By the tenth day the opening of the pituitary vesicle into the threat becomes 
 almost obliterated, and the lumen of the vesicle itself very much diminished. 
 The body itself consists of anastomosing cords of hyp 'blast-cells, the meso- 
 blast between which has already commenced to become vascular. The cords 
 or masses of hypoblast cells are surrounded by a delicate membrana propria, 
 and a few of them possess a small lumen. The infundibulum has increased in 
 length. 
 
 On the twelfth day the communication between the pituitary vesicle and 
 the throat is entirely obliterated, but a solid cord of cells still connects the 
 two. The vessels of the pia mater of the vesicle of the third ventricle have 
 become connected with the pituitary body, and the infundibulum has grown 
 down along its posterior border. 
 
 In the later stages, all connection is lost between the pituitary body and 
 the throat, and the former becomes connected with the elongated processes 
 infundibuli. 
 
 Such is Wilhelm Miiller's account* Goette , however (Archiv. Micr. Anat. 
 IX. p. 397), has recently given reasons for thinking that the pituitary diverti- 
 culum arises not from the closed foregut, lined \\ith hypoblast, but from the 
 buccttl cavity lined with epiblast. He states that in its earlier stages it may be 
 seen to start on the oral side of the partition, which for some time divides the 
 secondarily formed buccal cavity from the primarily formed foregut, and 
 therefore, belonging to the former, cannot be regarded as the natural anterior 
 termination of the latter. 
 
 Beyond an increase in size, which it shares with nearly 
 all parts of the embryo, and the change of position to which 
 we have already referred, the mid-brain undergoes no great 
 alteration during the third day. Its roof will ultimately 
 become developed into the corpora bigemina or optic lobes 
 (quadrigemina in mammals), its floor will form the crura 
 cerebri, and its cavity will be reduced to the narrow canal 
 known as the iter a tertio ad quartum ventriculum. 
 
 In the hind-brain, or third cerebral vesicle, that part 
 which lies nearest to the mid-brain, becomes during the 
 third day marked off from the rest by a slight constric- 
 tion. This distinction, which becomes much more evident- 
 later on by a thickening of the walls and roof of the front 
 portion, separates the hind-brain into the cerebellum in front, 
 and the medulla oblongata behind. While the walls of the 
 cerebellar portion of the hind -brain become very much 
 thickened as well at the roof as at the floor and sides, the 
 roof of the posterior or medulla oblongata portion thins out 
 into a mere membrane, forming a delicate covering to the 
 cavity of the vesicle (Fig. 26. iv), which here becoming 
 broad and shallow with greatly thickened floor and sides, is 
 
94 THE THIRD DAY. [CHAP. 
 
 known as the fourth ventricle, subsequently overhung by the 
 largely developed posterior portion of the cerebellum. 
 
 The third day, therefore, marks the distinct differentiation 
 of the brain into its fundamental parts: the cerebral hemi- 
 spheres, the central masses round the third ventricle, the 
 corpora bigemina, the cerebellum and the medulla oblongata; 
 the original cavity of the neural canal at the same time, 
 passing from its temporary division of three single cavities 
 into the permanent arrangement of a series of connected 
 ventricles, viz. the lateral ventricles, the third ventricle, the 
 iter (with a prolongation into the optic lobe on each side), 
 and the fourth ventricle. 
 
 10. At the same time that the outward external shape 
 of the brain is thus being moulded, internal changes are 
 taking place in the whole neural canal. These are best seen 
 in sections. 
 
 At its first formation, the section of the cavity of the 
 neural canal is round or nearly so. 
 
 About this time, however, the lining of involuted epiblast 
 along the length of the whole spinal cord becomes very much 
 thickened at either side, while increasing but little at the 
 mid-points above and below. The result of this is that the 
 cavity as seen in section (Fig. 44), instead of being circular, 
 has become a narrow vertical slit, almost completely filled in 
 on either side. 
 
 In the region of the brain the thickening of the lining 
 epiblast follows a somewhat different course. While almost 
 everywhere the sides and floor of the canal are greatly thick- 
 ened, the roof in the region of the various ventricles, not of 
 the fourth only, but of the others as well, becomes excessively 
 thin, so as to form a membrane reduced to almost a single 
 layer of cells. (Fig. 26. IV.) 
 
 11. In the preceding chapter we saw how the first cere- 
 bral vesicle, by means of lateral outgrowths followed by 
 constrictions, gave rise to the optic vesicles. These and the 
 parts surrounding them undergo on the third day changes 
 which result in the formation of the eyeball. 
 
 At their first appearance the optic vesicles stand out at 
 nearly right angles to the long axis of the embryo (Fig. 15), 
 and the stalks which connect them with the fore-bram are' 
 short and wide. We have already said (p. 77) that the con- 
 
THE OPTIC VESICLES. 
 
 95 
 
 strictions. which give rise to the stalks take place chiefly from 
 above downwards, and also somewhat inwards and backwards. 
 Thus from the first the vesicles appear to spring from the 
 under part of the fore-brain. 
 
 FIG. 26. 
 
 cc 
 
 jiOA 
 
 SECTION THROUGH THE HIND-BRAIN OF A CHICK AT THE END OF THE THIRD 
 DAY OF INCUBATION. 
 
 IV Fourth ventricle. The section shews the very thin roof and thicker sides 
 
 of the ventricle. 
 
 Ch. Notochord (diagrammatic shading). 
 
 CV. Anterior cardinal vein. 
 
 CC. Involuted auditory vesicle. CC points to the end which will form the 
 cochlear canal. RL. Kecessus labyrinth!, liy. hypoblast lining the alimen- 
 tary canal, hy is itself placed in the cavity of the alimentary canal, in that 
 part of the canal which will become the throat. The lower (anterior) wall of 
 the canal is not shewn in the section, but on each side are seen portions of a pair 
 of visceral arches. In each arch is seen the section of the aortic arch A OA 
 belonging to the visceral arch. The vessel thus cut through is running upwards 
 towards the head, being about to join the dorsal aorta AO. Had the section 
 been nearer the head, and carried through the plane at Which the aortic arch 
 curves round the alimentary canal to reach the mesoblast above it, A OA and A 
 would have formed one continuous curved space. In sections lower down in 
 the back the two aorta, A 0, one on either side would be found fused into one 
 median canal. 
 
 The shading of the mesoblast is diagrammatic ; it is here a uniform mass 
 of spindle-shaped cells ; there being in this region no differentiation into proto- 
 vertebre. 
 
96 THE THIRD DAY. [dlAP, 
 
 As the vesicles of the cerebral hemispheres grow out 
 rapidly from the front and under portions of the first cerebral 
 vesicle, they seem to thrust the optic vesicles apart and to 
 the sides. 
 
 Thus these, instead of standing out from the extreme 
 front, come to be placed at the sides of the head, the stalks, 
 which are correspondingly lengthened and narrowed, running 
 obliquely downwards and inwards from the vesicles to open 
 into the cavity of the brain at its base. Their openings are 
 at first placed close to each other at the junction of the 
 cerebral hemispheres with the remnant of the fore-brain (now 
 called the vesicle of the third ventricle), so that the cavities 
 of the two optic vesicles may be said to communicate directly 
 both with each other and with the cavities of the cerebral 
 hemispheres. The later connection is however soon lost, and 
 the stalks of the optic vesicles then open exclusively into the 
 third ventricle. At the same time the floor of the third ven- 
 tricle, during the occurrence of the cranial flexure, grows 
 down and thrusts apart the openings of the two optic stalks. 
 At a later date the stalks shift their position backwards, and 
 thus become connected chiefly with the base of the mid- 
 brain. 
 
 While these changes have been going on in the optic 
 stalks, development has also proceeded in the region of 
 the vesicles themselves, and given rise to the rudiments of 
 the retina, lens, vitreous humour, and other parts of the eye. 
 
 The changes through which these are formed are of a 
 somewhat complicated character, and not a few points in 
 reference to them are still involved in some doubt. 
 
 Towards the end of the second day, the external or super- 
 ficial epiblast which covers, and is in all but immediate 
 contact with the most projecting portion of the optic vesicle, 
 becomes thickened. This thickened portion is then driven 
 inwards in the form of a shallow open pit with thick walls 
 (Fig. 27 A. x), carrying before it the front wall (r) of the optic 
 vesicle. To such an extent does this involution of the super- 
 ficial epiblast take place, that the front wall of the optic 
 vesicle is pushed close up to the hind wall, and the cavity of 
 the vesicle becomes almost obliterated (Fig. 27, ). 
 
 The bulb of the optic vesicle is thus converted into a cup 
 with double walls, containing in its cavity the portion of 
 
V.] THE OPTIC CUP. 97 
 
 involuted epiblast. This cup, in order to distinguish its 
 cavity from that of the original optic vesicle, is generally 
 called the secondary optic vesicle. We may, for the sake of 
 brevity, speak of it as the optic cup ; in reality it never is a 
 vesicle, since it always remains widely open in front. Of its 
 double walls the inner or anterior (Fig. 27 B, r) is formed 
 from the front portion, the outer or posterior (Fig. 27 
 B, u) from the hind portion of the wall of the primary optic 
 vesicle. The inner or anterior (r), which very speedily 
 becomes thicker than the other, is converted into the retina ; 
 in the outer or posterior (u), which remains thin, pigment is 
 eventually deposited, and it ultimately becomes the tesselated 
 pigment-layer of the choroid. 
 
 FJG. 27. 
 A. B. 
 
 DIAGRAMMATIC SECTIONS ILLUSTRATING THE FORMATION OP THE EYE. 
 (After Eemak.) 
 
 In A, the thin superficial epiblast h is seen to be thickened at x, in front of 
 the optic vesicle, and involuted so as to form a pit o, the mouth of which 
 has already begun to close in. Owing to this involution, which forms the 
 rudiment of the lens, the optic vesicle is doubled in, its front portion r being 
 pushed against the back portion u, and the original cavity of the vesicle thus 
 reduced in size. The stalk of the vesicle is shewn as still broad. 
 
 In B, the optic vesicle is still further doubled in so as to form a cup with a 
 posterior wall u and an anteiior wall r. In the hollow of this cup lies the lens /, 
 now completely detached from the superficial epiblast x, h. Its cavity is still 
 shewn. The cavity of the stalk of the optic vesicle is already much narrowed. 
 
 By the closure of its mouth the pit of involuted epiblast 
 becomes a completely closed sac with thick walls and a small 
 central cavity. (Fig. 27 B, I). At the same time it breaks 
 away from the external epiblast, which forms a continuous 
 layer in front of it, all traces of the original opening being 
 lost. There is thus left lying in the cup of the secondary 
 optic vesicle, an isolated elliptical mass of epiblast. This is 
 E. 7 
 
98 THE THIRD DAY. [CHAP. 
 
 the rudiment of the lens. The small cavity within it speedily 
 becomes still less by the thickening of the walls, especially 
 of the hinder one. 
 
 At its first appearance the lens is in immediate contact 
 with the anterior wall of the secondary optic vesicle (Fig. 
 27 .B). In a short time however, the lens is seen to lie in 
 the' mouth of the cup (Fig. SOD), a space (vh) (which is 
 subsequently occupied by the vitreous humour) making its 
 appearance between the lens arid anterior wall of the vesicle. 
 
 In order to understand how this space is developed, the 
 position of the optic vesicle and the relations of its stalk 
 must be borne in mind. 
 
 FIG. 28. 
 
 DIAGRAMMATIC SECTION OF THE EYE AND THE OPTIC NERVE AT AN 
 EARLY STAGE (from Lieberkiihn), 
 
 to shew the lens I occupying the whole hollow of the optic cup, the inclination of 
 the stalk s to the optic cup, and the continuity ol the cavity of the stalk s' with 
 that of the primary vesicle c ; r, anterior, u posterior wall of the optic cup. 
 
 The vesicle lies at the side of the head, and its stalk is 
 directed downwards, inwards and backwards. The stalk in 
 fact slants away from the vesicle. Hence when the involu- 
 tion of the lens takes place, the direction in which the front 
 wall of the vesicle is pushed in is not in a line with the axis 
 of the stalk as for simplicity's sake has been represented in 
 the diagram Fig. 27, but forms an obtuse angle with that 
 axis, after the manner of Fig. 28, where s represents the 
 cavity of the stalk leading away from the almost obliterated 
 cavity of the primary vesicle. 
 
 Fig. 28 represents the early stage at which the lens fills 
 the whole cup of the secondary vesicle. The subsequent 
 state of affairs is brought about through the growth of the 
 
THE CHOROIDAL FISSURE. 
 
 99 
 
 walls of the cup taking place more rapidly than that of the 
 lens, or in other words to the cavity of the cup dilating. 
 But this growth or this dilatation does not take place equally 
 in all parts, of the cup. The walls of the cup rise up all 
 round except that point of the circumference of the cup 
 which is opposite the middle (from side to side) of the stalk. 
 While elsewhere the walls increase rapidly in height, carrying 
 so to speak the lens with them, at this spot, which in the 
 natural position of the eye is on its under surface, there is 
 no growth : the wall is here imperfect, and a gap is left. 
 Through this gap, which afterwards receives the name of the 
 choroidal fissure, a way is open from the mesoblastic tissue 
 surrounding the optic vesicle and stalk into the interior of 
 the cavity of the cup. 
 
 DIAGRAMMATIC EEPRESENTATION OF THE EYE OF THE CHICK OF ABOUT THE 
 
 THIRD DAY AS SEEN WHEN THE HEAD IS VIEWED FROM UNDERNEATH AS 
 A TRANSPARENT OBJECT. 
 
 / the lens, I' the cavity of the lens, lying in the hollow of the optic cup. 
 
 r the anterior, u the posterior wall of the optic cup, c the cavity of the 
 primary optic vesicle, now nearly obliterated. By inadvertence u has been 
 drawn in some places thicker than r, it should have been thinner through- 
 out. 
 
 s the stalk of the optic cup with s' its cavity, at a lower level than the cup 
 itself and therefore out of focus ; the dotted line indicates the continuity of 
 the cavity of the stalk with that of the primary vesicle. 
 The line 2, 2, through which the section shewn in Fig. 30 F is supposed 
 
 to be taken, passes through the choroidal fissure. 
 
 From the manner of its formation the gap or fissure is 
 evidently in a line with the axis of the optic stalk, and in 
 
100 THE THIED DAY. [CHAP. 
 
 order to be seen must be looked for on the under surface 
 of the optic vesicle. In this position it is readily recog- 
 nized in the transparent embryo of the third day, Figs. 25 
 and 29. 
 
 Bearing in mind these relations of the gap to the optic 
 stalk, the reader will understand how sections of the optic 
 vesicle at this stage present very different appearances 
 according to the plane in which the sections are taken. 
 
 When the head of the chick is viewed from underneath 
 as a transparent object the eye presents very much the ap- 
 pearance represented in the diagram Fig. 29. 
 
 D. Diagrammatic section taken perpendicular to the plane of the paper, 
 along the line y t y, Fig. 29. The stalk is not seen, the section falling 
 quite out of its region, vh, hollow of optic cup filled with vitreous 
 humour; other letters as in Fig. -27 B. 
 
 E. Section taken parallel to the plane of paper through Fig. 29, so far behind 
 the front surface of the eye as to shave off a small portion of the posterior 
 surface of the lens I, but so far in front as not to be carried at all through 
 the stalk. Letters as before ; /, the choroidal fissure. 
 
 F. Section along the line z, z, perpendicular to the plane of the paper, to shew 
 the choroidal fissure /, and the continuity of the cavity of the optic stalk 
 with that of the primary optic vesicle. Had this section been taken a 
 little to one side of the line 2, z, the wall of the optic cup would have 
 extended up to the lens below as well as above. Letters as before. 
 
 A section of such an eye taken along the line y, per- 
 pendicular to the plane of the paper, would give a figure 
 corresponding to that of Fig. 30 D. The lens, the cavity 
 and double walls of the secondary vesicle, the remains of the 
 primary cavity, would all be represented (the superficial 
 epiblast of the head would also be shewn) ; but there 
 would be nothing seen of either the stalk or the fissure. 
 If on the other hand the section were taken in a plane 
 parallel to the plane of the paper, at some distance above the 
 
V.] THE - CHOROID^L > FtSSURK. 
 
 101 
 
 level of the stalk, some such' gg^e:wcitrd' 5 b e''-gai^e3 as that 
 shewn in Fig. 30 E. Here the fissure / is obvious, and the 
 communication of the cavity vh of the secondary vesicle with 
 the outside of the eye evident; the section of course wo aid 
 not go through the superficial epiblast. Lastly, a section, 
 taken perpendicular to the plane of the paper along the line 
 z, i. e. through the fissure itself, would present the ap- 
 pearances of Fig. 30 F, where the wall of the vesicle is 
 entirely wanting in the region of the fissure marked by the 
 position of the letter f. 
 
 The fissure such as we have described it exists for a short 
 time only. Its lips come into contact, and unite (in the 
 neighbourhood of the lens, directly, but in the neighbourhood 
 of the stalk, by the intervention of a structure which we 
 shall describe presently), and thus the cup-like cavity of the 
 secondary optic vesicle is furnished with a complete wall 
 all round. The interior of the cavity is filled by the 
 vitreous humour, a clear fluid in which are a few scat- 
 tered cells. 
 
 In the foregoing account of the formation of the secondary optic vesicle, 
 and of the fissure, as the results of a process of unequal growth, we have fol- 
 lowed the account of Lieberkiihn (Uber das Auge des Wirbelthier embryos, 
 Schriften der Geseilsehaft zur Betbrderung der gesammten Naturwisserischatten 
 zu Marburg. Bd. 10. 1872). Their origin is more generally described as 
 being due to a doubling up of the primary vesicle from the side along the 
 line of the fissure at the same time that the lens is being thrust in in front. In 
 mammalia, the doubling up is said to involve the optic stalk, which becomes 
 flattened (whereby its original cavity is obliterated) and then folded in on itself, 
 so as to embrace a new central cavity continuous with the cavity of the 
 vitreous humour. 
 
 According to Lieberkiihn the optic stalk in birds is never so folded up, 
 but is converted into the optic nerve by the gradual obliteration of its primary 
 central cavity through increased thickening of the walls. The optic nerve 
 of the bird, moreover, contains no arteria centralis retina, while the involu- 
 tion of the optic stalk into the optic nerve was supposed to have for its purpose 
 the introduction of a quantity of mesoblast into the interior of nerve, in order 
 to form the artery. 
 
 According to Remak and the majority of observers after him, no mesoblast 
 whatever exists between the external epiblast and the optic vesicle, at the 
 point where the former is thrust inwards to form the lens, and hence this 
 organ carries with it in its involution no mesoblast whatever to serve as a 
 rudiment of either the vitreous humour or the capsule of the lens. They 
 described the vitreous humour as being formed entirely out of the meso- 
 blast which was intruded from the exterior of the eye through the choroidal 
 fissure, and Kolliker considered the capsule of the lens as a sort of cuticular 
 excretion from the surface of the lens itself. Lieberkiihn on the other hand 
 states that shortly after the commencement of the involution of the lens there 
 
102 v< THE T&IRD DAY. [CHAP. 
 
 may be^alraady ^V^lt^tli^n^y^r'Qf-me^oblaRt, interposed between it and the 
 optic vesicle. This layer is carried inward during the involution, and from 
 it both the vitreous humour and the capsule of the lens take their origin. 
 Jn birds it is very difficult to be sure of the existence of this layer, though 
 Lieberkiihn says that in mammals it is conspicuous ; and even if its existence 
 be admitted, it still remains doubtful whether it gives rise to the whole vi- 
 treous humour, or to the capsule of the lens only; though the latter view is most 
 probable. 
 
 During the changes in the optic vesicle just described, the 
 surrounding mesoblast takes on the characters of a distinct 
 investment, whereby the outline of the eyeball is definitely 
 formed. The internal portions of this investment, nearest to 
 the retina, become the choroid (i.e. the chorio-capillaris, 
 and the lamina fusca, the pigment epithelium, as we have 
 seen, being derived from the epiblastic optic cup), and pig- 
 ment is subsequently deposited in it. The remaining external 
 portion of the investment forms the sclerotic. 
 
 The complete differentiation of these two coats of the eye 
 does not however take place till a late period. Along the 
 line of the choroidal fissure the pigment is wanting. Con- 
 sequently in embryos of an age when the pigment has be- 
 come generally deposited in the choroid, a colourless streak 
 marking out the position of the choroidal fissure is very con- 
 spicuous. 
 
 In front of the optic cup the mesoblastic investment 
 grows forwards, between the lens and the superficial epi- 
 blast, and so gives rise to the substance of the cornea; the 
 epi blast supplying only the anterior epithelium. 
 
 At first the whole space between the lens and the super- 
 ficial epiblast is occupied by undifferentiated mesoblast; but 
 on the sixth day a layer of epithelium makes its ap- 
 pearance in midst of the mass, and thus divides it into an 
 anterior and a posterior portion. The anterior portion, in- 
 creasing in solidity, becomes the cornea, and remains con- 
 tinuous with the sclerotic; the epithelium in question per- 
 sisting as the posterior epithelium of the membrane of 
 Descemet. The posterior portion is reduced to a mere 
 membrane forming, according to Lieberkiihn, the front 
 limb of the capsule (and the suspensory ligament) of the 
 lens, the space between it and the cornea becoming filled 
 with aqueous humour. 
 
 We left the original cavity of the primary optic vesicle as 
 
V.] THE OPTIC CUP. 103 
 
 a nearly obliterated space between the two walls of the optic 
 cup. By the end of the third day the obliteration is com- 
 plete, and the two walls are in immediate contact. 
 
 The inner or anterior wall is, from the first, thicker than 
 the outer or posterior ; and over the greater part of the cup 
 this contrast increases with the growth of the eye, the 
 anterior wall becoming markedly thicker and undergoing 
 changes of which we shall have to speak directly (Fig. 31). 
 
 In the front portion however, along, so to speak, the 
 lip of the cup, anterior to a line which afterwards becomes 
 the ora serrata, both layers not only cease to take part in 
 the increased thickening, accompanied by peculiar histo- 
 logical changes, which the rest of the cup is undergoing, 
 but also completely coalesce together. Thus a hind portion 
 or true retina is marked off from a front portion. 
 
 The front portion, accompanied by the choroid which 
 immediately overlays it, is behind the lens thrown into folds, 
 the ciliary ridges ; while further forward it bends in between 
 the lens and the cornea to form the iris. The original wide 
 opening of the optic cup is thus narrowed to a smaller 
 orifice, the pupil ; and the lens, which before lay in the open 
 mouth of, is now inclosed in the cavity of the cup. While in 
 the hind portion of the cup or retina proper, no deposit of 
 black pigment takes place in the layer formed out of the 
 inner or anterior wall of the vesicle, in the front portion we 
 are speaking of, pigment is largely deposited throughout both 
 layers, so that eventually this portion seems to become 
 nothing more than a forward prolongation of the pigment- 
 epithelium of the choroid. 
 
 Thus while the hind moiety of the optic cup becomes the 
 retina proper, including the choroid-pigment in which the 
 rods and cones are imbedded, the front moiety is converted 
 into the ciliary portion of the retina, covering the ciliary 
 processes, and into the uvea of the iris ; the bodies of the 
 ciliary processes and the substance of the iris, their vessels, 
 muscles, connective tissue and ramified pigment, being derived 
 from the mesoblastic choroid. The margin of the pupil marks 
 the extreme lip of the optic vesicle, where the outer or poste- 
 rior wall turns round to join the inner or anterior. 
 
 We have still to speak of the choroidal fissure. In 
 mammals the slit remains open for a short time only. After 
 
104 
 
 THE THIRD DAY. 
 FIG. 31. 
 
 [CHAP. 
 
 p.Ch 
 
 SECTION OP THE EYE OF CHICK AT THE FOURTH DAT. 
 
 e. p. superficial epiblast of the side of the head. 
 
 R. true retina : anterior wall of the optic eup. p. Ch. pigment-epithelium of 
 
 the choroid : posterior wall of the optic cup. b is placed at the extreme 
 
 lip of the optic cup at what will become the margin of the pupil. 
 /. the lens. The hind wall, the nuclei of whose elongated cells are shewn at 
 
 nl, now forms nearly the whole mass of the lens, the front wall being 
 
 reduced to a layer of flattened cells el. 
 m. the mesoblast surrounding the optic cup arid about to form the choroid and 
 
 sclerotic. It is seen to pass forward between the lip of the optic cup and 
 
 the superficial epiblast. 
 
 Filling up a large part of the hollow of the optic cup is seen a hyaline mass 
 vh, possibly the rudiment of the vitreous humour. It has fallen away from 
 the retina at #, and is also (apparently accidentally) wanting at y. In the 
 neighbourhood of the lens it seems to be continuous as at cl with the tissue a, 
 which in turn is continuous with the mesoblast m, and appears to be the 
 rudiment of the capsule of the lens and suspensory ligament. 
 
V.] THE RETINA. 105 
 
 the formation of the vitreous humour within the cup, the 
 edges of the slit grow completely together, and all traces of 
 the seam disappear. In birds the course of events is some- 
 what different. 
 
 In addition to such amount of mesoblast as may pass 
 through the slit to form the vitreous humour, two special 
 processes of mesoblast grow in, one in the neighbourhood of 
 the optic stalk, in the region of the true retina, and a second, 
 which speedily becomes highly vascular, in that portion of 
 the slit which corresponds to the ciliary part of the retina. 
 The former piocess remains as the pecten so characteristic 
 of the avian eye, while the latter vascular process serves to 
 supply the pecten with blood. 
 
 By the twelfth day the fissure completely closes up and 
 disappears between these two processes and also in front of 
 the vascular one; but both the pecten and the vascular process 
 are left projecting into the interior of the eye. Hence in the 
 adult eye, the pecten seems to perforate the retina close to 
 the entrance of the optic nerve, the nervous fibres of the 
 retina spreading away in a radiate manner from it. 
 
 The optic stalk, which, as we have said, by an obliteration 
 of its central canal becomes converted into the optic nerve, is 
 at first equally continuous with the inner and with the outer 
 wall of the retina. This must of necessity be the case, since 
 the interval which primarily exists between the two walls 
 is continuous with the cavity of the stalk (vide Figs. 28 
 and 30 F, s). When the fibres however make their appear- 
 ance in the nerve, they are found to be connected with the 
 inner wall, or functional retina, only. 
 
 The histological condition of the eye in its earliest 
 stages is very simple. Both the epiblast forming the walls of 
 the optic vesicle, and the superficial layer which is thickened 
 to become the lens, are composed of several layers of simple 
 columnar cells. The surrounding mesoblast is made up of 
 cells whose protoplasm is more or less branched and irregu- 
 lar. These simple elements are gradually modified into the 
 complicated tissues of the adult eye, the changes undergone 
 being most marked in the cases of the retina, the optic nerve, 
 and the lens with its appendages. 
 
 The retina. At first the two walls of the optic cup do not 
 greatly differ in thickness. On the third day the outer or 
 
106 THE THIRD DAY. [CHAP. 
 
 posterior becomes much thinner than the inner or anterior, 
 and by the middle of the fourth day is reduced to a single 
 layer of flattened cells (Fig. 31, p. Ch.). At about the 80th 
 hour its cells commence to receive a deposit of pigment, and 
 eventually form the so-called pigmentary epithelium of the 
 choroid ; from them no part of the true retina (or no other 
 part of the retina, if the pigment-layer in question be sup- 
 posed to belong more truly to the retina than to the choroid) 
 is derived. 
 
 On the fourth day, the inner (anterior) wall of the optic 
 cup (Fig. 31, E) is perfectly uniform in structure, being 
 composed of elongated somewhat spindle-shaped cells, with 
 distinct nuclei. On its external (posterior) surface a distinct 
 cuticular membrane, the membrana limitans externa, early 
 appears. 
 
 As the wall increases in thickness, these cells multiply 
 rapidly, so that the wall becomes several cells thick. 
 
 The first indications of a division into layers are noticed 
 on the seventh day ; and on the eighth day a layer of 
 'granules' is very obvious. The granules, which are appar- 
 ently nuclei of cells, become on the tenth day distinctly 
 arranged into an inner and an outer layer ; and at about 
 the same time small processes, apparently outgrowths from 
 the outer granular layer, make their appearance on the 
 external surface of the membrana limitans externa. These 
 processes are the rudiments of the rods and cones. 
 
 From the first they may be roughly divided into two categories, (i) those of 
 smaller, (2) those of larger diameter. Both kinds grow rapidly and in the tips 
 of both small highly refractive globules soon appear. The thinner processes 
 are the cones, the thicker the rods. The cones remain for a long time thinner 
 than the rods, but shortly before the exclusion of the chick they increase rapidly 
 in diameter and soon after that occurrence are found to surpass the rods in 
 thickness. On the i8th day some of the globules in the cones become 
 red, on the iQth others become yellow, and very soon all the globules in the 
 cones acquire a distinct colour. The globules in the rods remain uncoloured. 
 The rods and cones then are outgrowths through the membrana limitans externa, 
 from the inner wa 1 ! of the optic cup or retina into the outer wall or pigment- 
 epithelium of the choroid. 
 
 Remak and some other investigators were of opinion that the outer wall of 
 the optic cup gave rise to the rods and cones as well as to the pigment-epi- 
 thelium. The observations however of Max Schultze, Archiv Micros. Anat IV. 
 ]>. 239, supported by Babuchin, Wurz. Nat. Zeifsch. TV. (1863) p. 71, and others, 
 Lave clearly shewn that Remak's views were erroneous. 
 
 On the thirteenth day the molecular layer and the gan- 
 
V.] THE OPTIC NERVE. 107 
 
 glionic layer are distinguishable. Very early the substance of 
 certain of the cells takes on the appearance of fibres, arranged 
 vertically, i. e. radiating from the inner or anterior surface of 
 the retina to the membrana limitans externa. These are 
 the rudiments of the fibres of Miiller. 
 
 Thus of the cells of the inner wall of the cup, some 
 becoine ganglionic cells, and others the fibres of Miiller, 
 while the nuclei of yet others remain as the inner and outer 
 granules. The rods and cones are outgrowths of the cells to 
 which the outer granules belong. All pa.rts of the retina, 
 in fact, whether simply connective, or really nervous in nature, 
 seem to be derived from epiblastic cells. 
 
 The changes described above are confined to that portion 
 of the retina which lies behind the ora serrata. In front of 
 this both walls of the cup coalesce as we have said into a 
 cellular layer in which a deposit of pigment takes place. 
 
 The optic nerve. Histological changes are first observable 
 in the optic stalk at about the time when its cavity loses all 
 connection with the cerebral hemisphere and opens ex- 
 clusively into the third ventricle.. It is then that fibres first 
 make their appearance in its walls, nuclei being still abun- 
 dantly present. The stalk though much elongated is still 
 hollow and its cavity is circular in section. According to 
 Lieberkiihn at no time does it (in the bird) undergo any 
 involution tending to obliterate its cavity. 
 
 Soon after the deposition of pigment in the outer wall of 
 the optic cup, while the optic stalks are as yet still hollow, 
 the rudiments of the optic chiasma appear. The fibres of 
 the one stalk grow over into the attachment of the other. 
 About the same time the fibres at the neck of the optic cup 
 grow forward and become connected with the retina, over whose 
 internal surface they spread. The entrance of the optic 
 nerve into the eyeball is closely connected with that of the 
 pecten, its fibres passing in at the lower end of that body, 
 coursing along its sides to its upper end and radiating from 
 it as from a centre to all parts of the retina. 
 
 Before the exclusion of the chick the optic nerve becomes 
 solid by the gradual filling up of its central cavity. 
 
 The lens when first formed is somewhat elliptical in 
 section with a small central cavity of a similar shape, 
 the front and hind walls being of nearly equal thickness, 
 
108 THE THIRD DAY. [CHAP. 
 
 each consisting of a single layer of elongated columnar 
 cells. 
 
 In the subsequent growth of the lens, the development of 
 the hind wall is of a precisely opposite character to that of 
 the front wall. The hind wall becomes much thicker, and 
 tends to obliterate the central cavity by becoming convex on 
 its front surface. At the same time its cells, still remaining 
 as a single layer, become elongated and fibre-like. The 
 front wall on the contrary becomes thinner and thinner and 
 its cells more and more flattened and pavement-like. 
 
 These modes of growth continue until at the end of the 
 fourth day, as shewn in Fig. 31, the central hind wall I is in 
 absolute contact with the front wall el and the cavity thus 
 becomes entirely obliterated. The cells of the hind wall have 
 by this time become veritable fibres which when seen in 
 section appear to be arranged nearly parallel to the optic 
 axis, their nuclei nl being seen in a row along their middle. 
 The front wall, somewhat thickened at either side where 
 it becomes continuous with the hind wall, is now a single 
 layer of flattened ceils separating the hind wail of the lens, 
 or as we may now say the lens itself, from the front limb of 
 the lens-capsule ; of this it becomes the epithelium. 
 
 The subsequent changes undergone consist chiefly in 
 the continued elongation and multiplication of the lens-fibres, 
 with the partial disappearance of their nuclei. 
 
 During their multiplication they become arranged in the 
 manner so characteristic of the adult lens. 
 
 The lens-capsule and its adjuncts. In spite of the 
 numerous investigations which have been made in reference 
 to the development of the lens-capsule, its precise mode 
 of origin can hardly even yet be said to be certainly known. 
 Remak was led from analogy to regard it as a product of the 
 mesoblast, though he did not succeed in satisfactorily demon- 
 strating the fact. Kolliker looked upon it as a cuticular 
 membrane thrown off by the superficial cells of the lens, and 
 his view has been very generally adopted. 
 
 Lieberkiihn has given a different account of its origin. 
 According to him the involution of the lens, as we have already 
 stated, carries inwards with it a very thin layer of meso- 
 blast. This remains continuous with the mesoblast surround- 
 ing the eyeball, so that when subsequently the mesoblast 
 
V.] THE LENS-CAPSULE. 109 
 
 grows forward over the front of the lens, the latter receives 
 a complete mesoblastic investment. 
 
 Of this mesoblast a very thin layer next to the lens 
 both in front and behind becomes separated from the 
 rest, and forms the lens-capsule and suspensory ligament. 
 The remainder of the mesoblast behind the lens becomes 
 converted into the vitreous humour, the layer immediately 
 in contact with the retina giving rise to the hyaloid 
 membrane. That the hyaloid is really a product of the 
 mesoblast and not a cuticular outgrowth from the epiblastic 
 cells of the retina is indicated by the fact that it is con- 
 tinuous over the pecten, where of course the retina is absent. 
 At its first appearance the vitreous humour is a mass of 
 stellate cells ; while however it is rapidly enlarging to fill 
 up the growing optic cup, a large portion of it becomes 
 entirely fluid, the cellular elements being more and more 
 restricted to the immediate neighbourhood of the posterior 
 surface of the lens, where a few stellate cells may be seen 
 even in the adult. 
 
 Briefly to recapitulate. The eye commences as a lateral 
 outgrowth of the fore-brain, in the form of a stalked vesicle. 
 
 The stalk becoming narrowed and subsequently solid, is 
 converted into the optic nerve. 
 
 An involution of the superficial epiblast over the front of 
 the optic vesicle, in the form first of a pit, then of a closed sac 
 with thick walls, and lastly, of a solid rounded mass (the small 
 central cavity being entirely obliterated by the thickening of 
 the hind wall), gives rise to the lens. Owing to this involu- 
 tion of the lens, the optic vesicle is doubled up on itself, and its 
 cavity obliterated ; thus a secondary optic vesicle or optic cup 
 with a thick anterior and a thin posterior wall is produced. As 
 a result of the manner in which the doubling up takes place, 
 or of the mode of growth afterwards, the cup of the secondary 
 optic vesicle is at first imperfect along its under surface, where 
 a gap, the choroidal fissure, exists for some little time, but 
 subsequently closes up. 
 
 The mesoblast in which the eye is imbedded gathers 
 itself together around the optic cup into a distinct invest- 
 ment of which the internal layers become the choroid, the 
 external, the sclerotic. An ingrowth of this investment 
 between the front surface of the lens and the superficial 
 
110 
 
 THE THIRD DAY. 
 
 [CHAP. 
 
 epiblast furnishes the body of the cornea, the epiblast itself 
 remaining as the anterior corneal epithelium. 
 
 A portion of mesoblast, carried in from the front by the 
 lens during its involution, gives rise to the capsule of the 
 lens and suspensory ligament, while some mesoblast entering 
 on the under side through the choroidal fissure becomes (in 
 birds) the pecten, and probably also contributes to the 
 vitreous humour. 
 
 Of the walls of the optic cup, the thinner outer (posterior) 
 wall becomes, behind the line of the ora serrata, the pigment- 
 epithelium of the choroid, while the thicker inner (anterior) 
 wall supplies all the elements of the retina, including the 
 rods and cones which grow out from it into the pigment- 
 epithelium. 
 
 FIG. 32. 
 
 AOA 
 
 SECTION THROUGH THE HIND-BRAIN OP A CHICK AT THE END OF THE THIRD 
 
 DAY OF INCUBATION. 
 IV. Fourth ventricle. The section shews the very thin roof and thicker sides 
 
 of the ventricle. 
 
 Ch. Notochord (diagrammatic shading). 
 CV. Anterior cardinal vein. 
 
 CO. Involuted auditory vesicle. CC points to the end which will form the 
 cochlear canal. ML. Kecessus labyrinthi. Jiy. Hypoblast lining the 
 alimentary canal. AO, AOA, Aorta, and aortic arch. 
 
V.] THE EAR. Ill 
 
 In front of the line of the ora serrata, both walls of the 
 optic cup, quite thin and wholly fused together, give rise to 
 the pigment-epithelium of the ciliary processes and iris, the 
 bodies of both these organs being formed from the meso- 
 blastic investment. 
 
 12. During the second day the ear first made its appear- 
 ance, on either side of the hind-brain as an involution of the 
 external epi blast, thrust down into the mass of mesoblast 
 rapidly developing between the epiblast of the skin and that of 
 the neural canal (Fig. -15, au.p.). It then had the form of a 
 shallow pit with a widely open mouth. Before the end of the 
 third day, its mouth closes up and all signs of the opening are 
 obliterated. The pit thus becomes converted into a closed 
 vesicle lined with epiblast and surrounded by mesoblast. 
 This vesicle is the otic vesicle, whose cavity rapidly enlarges 
 while its walls become thickened (Fig. 32, (7(7). 
 
 The changes by which this simple otic vesicle is converted 
 into the complicated system of parts known as the internal 
 ear, have been much more completely worked out for 
 mammals than for birds. We shall therefore reserve a full 
 account of them for a later portion of this work. Meanwhile a 
 brief statement of the main course of events in the chick may 
 be useful ; and will be most conveniently introduced here, 
 although we shall have, in doing so, to speak of changes 
 which do not occur till much later than the third day. 
 
 The internal ear consists essentially of an inner mem- 
 branous labyrinth lying loosely in and only partially attached 
 to an outer osseous labyrinth. 
 
 The membranous labyrinth (Fig. 33) consists of two 
 parts: (1) the vestibule, with which are connected three pairs 
 of semicircular canals, pag', fr, hor, and a long narrow 
 hollow process, the aqueductus or recessus vestibuli, and 
 (2) the ductus cochlearis, which in birds is a flask-shaped 
 cavity slightly bent on itself. The several parts of each of 
 these cavities freely communicate, and the two are joined 
 together by a narrow canal, the canalis reuniens, cr. 
 
 The osseous labyrinth has a corresponding form, and may 
 be similarly divided into parts : into a bony vestibule, with 
 its bony semicircular canals and recessus vestibuli, and into 
 a bony cochlea ; but the junction between the cochlea and the 
 bony vestibule is much wider than the membranous canalis 
 reuniens. 
 
112 
 
 THE THIRD DAY. 
 
 FIG. 33. 
 
 B 
 
 [CHAP. 
 
 -pay' 
 
 TWO VIEWS OP THE MEMBRANOUS LABYRINTH OF COLUMBA DOMESTICA (copied 
 from Hasse). A from the exterior, B from the interior. 
 
 hoi'' horizontal semicircular canal, hor ampulla of ditto, pag' posterior vertical 
 semicircular canal, pag ampulla of ditto, /?' anterior vertical semicircular 
 canal, fr ampulla of ditto, u utriculus, ru recessus utriculi, v the connect- 
 ing tube between the ampulla of the anterior vertical semicircular canal and 
 the utriculus, de ductus endolymphaticus (recessus vestibuli), s sacculus 
 hemisphericus (this is smaller in birds than in any other vertebrate), cr 
 canalis reuniens, lag lagena (the dilated extremity of the cochlea), mr 
 membrane of Reissner, which forms the boundary between the scala 
 vestibuli and scala media, pb Basilar membrane, which forms the boundary 
 between the scala tympani and the scala media. 
 
 The cochlea of a bird consists ( r) of a scala vestibuli with a very small lumen, 
 which opens at one end into the perilymphatic cavity of the vestibule, and at 
 the other into the lagena (the dilated extremity of the cochlea corresponding 
 with the cupola of mammals) ; (21 of a scala tympani, also opening into the 
 lagena at one end, and into the foramen rotundum at the other; (3) of a 
 scala media ending blindly at one end, but in communication with the mem- 
 branous vestibule at the other, through the membranous canalis reuniens (cr). 
 
 As in mammals, the cavity of the osseous cochlea is 
 divided lengthways by the ductus cochlearis into a scala 
 tympani ending in a foramen rotundum, and a scala vestibuli 
 ending in the cavity of the osseous vestibule, which in its 
 turn is connected with the foramen ovale. 
 
 The auditory nerve, piercing the osseous labyrinth in 
 various points, is distributed in the walls of the membranous 
 labyrinth. 
 
 All these complicated structures are derived from the 
 simple primary otic vesicle by changes in its form and 
 differentiation of its walls. All the epiblast of the vesicle 
 
THE MEMBRANOUS LABYRINTH. 
 
 113 
 
 goes to form the epithelium of the membranous labyrinth, 
 whose curiously twisted cavity filled with endolymph repre- 
 sents the original cavity which was first open to the surface 
 but subsequently covered in. The corium of the mem- 
 branous labyrinth, and all the tissues of the osseous labyrinth, 
 
 FIG. 34. 
 
 R.Z7 
 
 V:B 
 
 TRANSVERSE SECTION OF THE HEAD OF A FCETAL SHEEP (16 MM. IN LENGTH) 
 IN THE REGION OF THE HIND BRAIN. (Copied from Boettcher, Entwicke- 
 lung und Bau des Gehorlabyrinths.) 
 
 This figure, together with Figs. 33 and 34, though referring to mammalian 
 structures, are introduced here, in order to illustrate the account given in the 
 text. 
 H.B. the hind brain, the lines of reference starting on both sides from the 
 
 greatly thickened side walls. 
 
 The section is somewhat oblique, hence while on the right side the connec- 
 tions of the recessus vestibuli, R.L., and of the commencing vertical semicircular 
 canal V.B. and of the ductus cochlearis C. C., with the cavity of the primary 
 otic vesicle are well seen, on the left side, only the extreme end of the ductus 
 cochlearis C.C. and of the semicircular canal V.B. are shewn. In the same way 
 the cavity of the throat appears from the obliquity of the section to be one-sided. 
 
 Lying close to the inner side of the otic vesicle is seen the cochlear 
 ganglion G. C. ; on the left side the auditory nerve G and its connection N with 
 the hind-brain are also shewn. 
 
 Below the otic vesicle on either side lies the cardinal vein. 
 
 E. 
 
 8 
 
THE THIRD DAY. 
 
 [CHAP. 
 
 are developed out of the mesoblastic investment of the 
 vesicle, the (lymphatic) space between the osseous and 
 membranous labyrinths being hollowed out of the mesoblast, 
 and becoming filled with perilymph as it is formed. The chief 
 stages are as follows : 
 
 The form of the vesicle as seen in transverse sections 
 of the head from being oval becomes somewhat triangular 
 with the apex directed downwards (i. e. towards the ventral 
 surface of the body). (Fig. 32.) 
 
 The apex continues to develope until it becomes a some- 
 what pointed process directed inwards (Fig. 34, (7(7), and lying 
 somewhat in front (nearer the face than) the rest of the vesicle, 
 
 FIG. 35. 
 
 c.c 
 
 SECTION OF THE HEAD OF A FOETAL SHEEP (20 MM. IN LENGTH). 
 
 (Copied from Boettcher.) 
 
 ?. V. Recessus Vestibuli. V.B. Vertical semicircular canal. H.B. Horizontal 
 semicircular canal. C.C. Cochlear canal. G. Cochlear ganglion. 
 
V.] THE SEMICIRCULAR CANALS. 115 
 
 from which it soon becomes definitely marked off by a constric- 
 tion as the ductus cochlearis (Fig. 35, C. (7.), leaving the rest of 
 the vesicle to form the vestibule and its appendages. About the 
 same time that the ductus cochlearis is developing the upper 
 and outer corner of the triangular vestibule grows out up- 
 wards and backwards, as a long hollow process, the recessus 
 or aqueductus vestibuli (Fig. 35, -RF, 36, RV), marked 
 where it leaves the vestibule by a more* or less prominent 
 constriction. The upper end of this is the remnant of the 
 primitive opening of the auditory involution. 
 
 On the remaining surface of the vestibule two hollow 
 protuberances are visible (Fig. 34, V.B.], the rudiments of 
 the two vertical semicircular canals. Below this, a little later 
 on, a similar bulging starts to form the horizontal semi- 
 circular canal (Fig. 35, H.B.). Each protuberance grows 
 out, becomes flattened, and while remaining attached to the 
 vestibule at its two ends becomes separated from it in the 
 middle, and is thus converted into a single tube opening at 
 both ends (one of which subsequently dilates into an ampulla) 
 into the cavity of the vestibule. 
 
 Near the junction of the ductus cochlearis with the 
 
 vestibule, two constrictions give rise to a prominence lying 
 
 between them, and known as the sacculus hemisphericus. This, 
 
 though very conspicuous in mammals (Fig. 36, S.B.}, is very 
 
 slightly marked in birds (Fig. 33, s). The rest of the vesti- 
 
 i bule remains as the utriculus. The progressive constriction 
 
 I at the base of the ductus cochlearis gives rise to the canalis 
 
 i reuniens. Thus, by an unequal growth resulting in these 
 
 protuberances and constrictions, the originally oval sac is 
 
 I modelled into the membranous labyrinth. 
 
 While the vesicle is thus increasing in size and changing 
 in form, a large quantity of mesoblast is gathered round it. 
 The outer portion of this mesoblastic investment becoming 
 dense is converted into cartilage, while the internal portion 
 remains as undifferentiated tissue. Later on, the fnost in- 
 ternal layers of this undifferentiated tissue are converted 
 into the membranous coat (corium) which supports the 
 epithelium of the membranous labyrinth ; while the re- 
 mainder is liquefied or absorbed at the same time that the 
 cartilaginous envelope becomes ossified, and so gives rise to 
 the cavity of the osseous labyrinth with its contained peri- 
 
116 
 
 THE THIRD DAY. 
 FIG. 36. 
 
 [CHAP. 
 
 C/i- 
 
 ac 
 
 SECTION THROUGH THE INTERNAL EAR OF AN EMBRYONIC SHEEP (28 MM. 
 IN LENGTH). (Copied from Boettcher.) 
 
 DM. Dura Mater. R.V. Recessus vestibuli. H.V.B. Posterior vertical 
 semicircular canal. U. Utriculus. H.B. Horizontal semicircular canal. 
 b. Canalis reuniens. a. Constriction by means of which the sacculus 
 hemisphericus S.R. is formed. /. Narrowed opening between sacculus 
 hemisphericus and utriculus. 0.0. Cochlea. <7.<7. Lumen of cochlea. 
 K.K. Cartilaginous capsule of cochlea. K.B. Investing mass. Oh. 
 Chorda dorsalis. 
 
V.] THE NASAL PITS. 117 
 
 lymph. In the region of the cochlea this excavation of the 
 mesoblast takes place along two definite tracts, and thus the 
 two scalse are established ; but in the region of the vestibule 
 and its appendages it is more irregular, resulting in one 
 common cavity interrupted at various points by bridles of 
 connective tissue passing from the osseous to the membranous 
 labyrinth. 
 
 Further details, especially concerning the histological 
 changes, we propose to reserve for a later part of this 
 work. Meanwhile we may remark that all the minute 
 auditory apparatus, the hair-cells, and their various adjuncts, 
 appear to be of a distinctly epithelial nature and of epiblastic 
 origin. In the bird, as is well known, there are no rods of 
 Corti; but even these structures seem in the mammal to be 
 similarly epiblastic. 
 
 It will be seen then that the ear, while resembling the 
 eye in so far as the peculiar structures in which the sensory 
 nerve in each case terminates are formed of involuted epi- 
 blast, differs from it inasmuch as it arises by an independent 
 involution of the superficial epiblast, whereas the eye is a 
 constricted portion of the general involution which gives rise 
 to the central nervous system. Still greater is the distinction 
 between the optic and auditory nerves. The optic nerve is, 
 as we have seen, the consolidated stalk of the optic vesicle, 
 and therefore is of purely epiblastic origin. The auditory 
 nerve, on the contrary, as we shall see, arises in and is formed 
 out of the mesoblast, lying by the side of the otic vesicle. It 
 with its ganglion may readily be recognised in sections as 
 quite independent, both of the otic vesicle and the hind- 
 brain, though subsequently coming into connection with 
 each of them. The growth of the auditory nerve into the 
 hind-brain is shewn in Fig. 34, N; the union of the nerve- 
 fibres with the epithelial structures of the membranous 
 labyrinth takes place at a later date. 
 
 13. At the under surface of each of the vesicles of the 
 cerebral hemispheres there appears towards the end of the 
 third day a small spmewhat elongated vesicle, the olfactory 
 vesicle, which is the rudiment of the olfactory nerve or bulb. 
 Over each olfactory vesicle the external epiblast which covers 
 them grows inwards to form a shallow depression with a 
 thickened border. These depressions are the nasal pits (Fig. 
 
118 THE THIRD DAY. [CHAP. 
 
 37, N). Like the lens and the labyrinth of the ear, they are 
 
 HEAD OF AN EMBRYO CHICK OF THE THIRD DAY VIEWED SIDEWAYS AS AN 
 OPAQUE OBJECT. (Chromic acid preparation.) 
 
 C.H. Cerebral hemispheres. F. B. Vesicle of third ventricle. M.B. Mid-brain. 
 Cb. Cerebellum. H.B. Medulla oblongata. 
 
 N. Nasal pit. ot. otic vesicle in the stage of a pit with the opening not yet 
 closed up. op. Optic vesicle, with I. lens and ch.f. choroidal fissure. The 
 small dot in the centre of the lens indicates the remnant of its external 
 opening. The superficial epiblast moulds itself to the form of the optic 
 vesicle and the lens ; hence the choroidal fissure, though formed entirely 
 underneath the superficial epiblast, is distinctly visible from the outside. 
 
 T F. The first visceral fold ; above it is seen a slight indication of the superior 
 maxillary process. 
 
 2, 3, 4 F. Second, third and fourth visceral folds, with the visceral clefts 
 between them. 
 
 SO. Portion of the somatopleure springing from between the ends of the 
 visceral folds. 
 
 formed from the external epiblast; unlike them they are 
 never closed up. At first they have no distinct connection 
 with their respective olfactory vesicles, and their openings 
 are independent and separate, there being as yet no actual 
 mouth to connect them with each other. 
 
 14. It must be borne in mind that especially in the 
 early stages of development, owing, to the very unequal 
 growth of different parts, the relative position of the various 
 structures is continually shifting. This is very well seen in 
 the instance of the heart. At its first appearance, the heart 
 is lodged immediately beneath the extreme front of the ali- 
 mentary canal so far forwards as to underlie that portion of 
 the medullary canal which will form the brain. It is, in 
 fact, at that epoch a part of the head. From that early 
 position it gradually recedes farther and farther backward, 
 until, at the end of the third day, a considerable interval is 
 
V.] THE VISCERAL CLEFTS. 119 
 
 observed between it and the actual head. In other words, 
 a distinct neck has been formed, in which most important 
 changes take place. 
 
 The neck is distinguished from the trunk in which the 
 heart now lies by the important feature that in it there is no 
 cleavage of the mesoblast into somatopleure and splanch- 
 nopleure, and consequently no pleuroperitoneal cavity. In 
 passing from the exterior into the alimentary canal, the three 
 layers of the blastoderm are successively traversed without 
 any breach of continuity save such as is caused by the cavities 
 of the blood-vessels. In this neck, so constituted, there ap- 
 pear on the third day certain fissures or clefts, the visceral or 
 branchial clefts. These are real clefts or slits passing right 
 through the walls of the throat, and are placed in series on 
 either side across the axis of the alimentary canal, lying not 
 quite at right angles to that axis and parallel to each other, 
 but converging somewhat to the middle of the throat in front 
 (Fig. 37). Viewed from the outside in either fresh or pre- 
 served embryos they are not very distinctly seen to be clefts; 
 but when they are seen from within after laying open the 
 throat, their characters as elongated oval slits can easily be 
 recognised. 
 
 Four in number on either side, the most anterior is the 
 first to be formed, the other three following in succession. 
 Their formation takes place from within outwards. The 
 hypoblast and mesoblast are first absorbed along the line 
 of the future cleft, then the epiblast is broken through, and 
 the hypoblast, which is carried outwards as a lining to the 
 slit, forms a junction with the epiblast at the outside of the 
 throat. 
 
 No sooner has a cleft been formed than its upper border 
 (i. e. the border nearer the head) becomes raised into a thick 
 lip or fold, the visceral or branchial fold. Each cleft has its 
 own fold on its upper border, and in addition the lower 
 border of the fourth or last visceral cleft is raised into a 
 similar fold. There are thus five visceral folds to four 
 visceral clefts (Fig. 37). The last two folds however, and 
 especially the last, are not nearly so thick and prominent 
 as the other three, the second being the broadest and most 
 conspicuous of all. The first fold meets, or nearly meets its 
 fellow in the middle line in front, but the second falls short 
 
120 THE THIRD DAY. [CHAP. 
 
 of reaching the middle line, and the third, fourth and fifth 
 do so in an increasing degree. Thus in front views of the 
 neck a triangular space with its apex directed towards the 
 head is observed between the ends of the several folds. 
 
 Into this space the pleuroperitoneal cavity extends, the 
 somatopleure separating from the splanchnopleure along the 
 ends of the folds; and it is here that the aorta plunges into 
 the mesoblast of the body. 
 
 Of the history of these most important visceral folds and 
 clefts we shall speak in detail hereafter ; meanwhile we may 
 say that in the chick and higher vertebrates the first three 
 pairs of folds are those which call for most notice. 
 
 The first fold on either side, increasing rapidly in size 
 and prominence, does not, like the others, remain single, but 
 sends off in the course of the third day a branch or bud-like 
 process from its upper edge. This branch, starting from 
 near the outer beginning of the fold, runs forwards and 
 upwards, tending to meet the corresponding branch from the 
 fold on the other side, at a point in the middle line nearer 
 the front of the head than the junction of the main folds. 
 The two branches do not quite meefc, being separated by 
 a median process, which at the same time grows down 
 from the extreme front of the head, and against which they 
 abut. Between the main folds, which are directed somewhat 
 downwards and the branches which slant upwards, a some- 
 what lozenge-shaped space is developed which, as the folds 
 become more and more prominent, grows deeper and deeper. 
 The main folds are the rudiments of the inferior maxillcv, 
 the branches those of the superior maxillce, the lozenge- 
 shaped cavity between them is the cavity of the mouth, 
 and the descending process which helps to complete the 
 upper margin of this cavity is called, from the parts which 
 will be formed out of it, the fronto-nasal process. 
 
 Already on the second day the under wall of the front 
 end of the alimentary canal began to become thin, while over 
 it the epiblast was pushed in so as to form a depression. The 
 maxillary folds convert this depression into a deep pit, 
 whose bottom is not as yet perforated, the opening into 
 the alimentary canal being made later on. 
 
 The two succeeding pairs of visceral folds are transformed into parts which 
 will be best considered in connection with the development of the skull. The 
 
V.] THE AORTIC ARCHES. 121 
 
 last two disappear in the chick without giving rise to any permanent 
 structures. 
 
 The first visceral cleft remains permanently open, but is drawn out by the 
 growth of surrounding parts into a long tube, subsequently divided into the 
 rneatus auditorius and the Eustachian tube. The other visceral cleits are 
 obliterated. 
 
 15. By the end of the second day three pairs of aortic 
 arches had been established in connection with the heart. 
 When the visceral folds and clefts are formed, a definite 
 arrangement between them and the aortic arches is always 
 
 FIQ. 38. 
 
 THE SAME HEAD AS SHEWN IN FIG. 37, SEEN FROM THE FRONT. 
 
 The neck has been cut across between the first and second visceral folds, the 
 incision being carried through the first visceral cleft. In the cut surface b 
 are seen the sections of the hind brain Hb., and of blood-vessels c. 
 
 i F. The first visceral fold ; between the ends of the fold is seen a section of 
 the somatopleure at its extreme forward limit; in it lies the aorta . 
 Below the folds is the cavity of the throat a/, and /. Fis placed in the first 
 visceral cleft. 
 
 C.H. Cerebral hemispheres. N. Nasal pit. ch.f. groove indicating the 
 choroidal fissure. 
 
 observed. The first visceral cleft runs between the first and 
 second aortic arches. Consequently the first aortic arch runs 
 in the first visceral fold and the second in the second. In 
 the same way, the second visceral cleft lies between the 
 second and third aortic arches, the third aortic arch running 
 in the third visceral fold. Each aortic arch runs in the 
 thickened mesoblast of the corresponding fold. 
 
 Arrived at the upper surface of the alimentary canal, 
 these arches unite at acute angles to form a common trunk, the 
 dorsal aorta (Fig. 39 A, A. 0\ which runs along the back imme- 
 diately under the notochord. The length of this common single 
 trunk is not great, as it soon divides into two main branches 
 
122 
 
 [CHAP. 
 
 DIAGRAM OF THE ARTERIAL CIRCULATION ON THE THIRD DAY. 
 
 I, 2, 3. The first three pairs of aortic arches. A . The vessel formed byythe 
 junction of the three pairs of arches. A.O. Dorsal aorta formed by the 
 junction of the two branches A and A ; it quickly divides again into two 
 branches which pass down one on each side of the notochord. From each 
 of these, not far from its termination, is given off a large branch Of. A., 
 the omphalo-mesaraic artery. 
 
 (the future common iliacs), each of which, after giving off the 
 large omphalo-mesaraic artery, Of. A., pursues its course with 
 diminished calibre to the tail, where it is finally lost in the. 
 capillaries of that part. 
 
 16. The heart is now completely doubled up on itself. 
 Its mode of curvature is apparently somewhat complicated. 
 
V.] THE HEART. 123 
 
 Starting from the point of junction of the omphalo-mesaraic 
 veins (Fig. 24, Ht), there is first a slight curvature towards the 
 left ; this is followed by a turn to the right, and then the heart 
 is completely bent on itself, so that afterwards it pursues a 
 course directed from behind quite straight forwards (except 
 perhaps for a little inclination to the left) to the point where 
 the aortic arches branch off. In this way the end of the 
 bulbus arteriosus comes to lie just underneath (or in front of 
 according to the position of the embryo) that part which has 
 already been marked off by the lateral bulgings as the 
 auricular portion. 
 
 That part of the heart which is turned to the right, 
 including the point of doubling up, is the ventricular portion, 
 and is even at this stage separated from the auricular portion 
 by a slight neck. This external constriction corresponds to 
 an internal narrowing of the lumen of the heart, and marks 
 the position of the future canalis auricularis. 
 
 The ventricular portion is, on the other hand, likewise 
 separated by a fainter constriction from the anterior continua- 
 tion of the heart which forms the bulbus arteriosus. The 
 projecting part where the doubling takes place is at this 
 stage still quite round ; we shall see that later on it becomes 
 pointed and forms the apex of the heart. 
 
 The whole venous portion of the heart (if we may so 
 speak, though of course at this stage blood of the same 
 quality passes right along the whole cardiac canal) lies in a 
 plane which is more dorsal than the arterial portion. The 
 point at which the venous roots of the heart, i. e. the two 
 omphalo-mesaraic trunks unite into a single canal, is on this 
 day carried farther and farther away from the heart itself. 
 By the end of the day there is a considerable distance 
 between the auricular portion of the actual heart and the 
 point where the venous roots separate each to pursue its 
 course along the splanchnopleure-fold of its own side. This 
 distance is traversed by a single venous trunk, of which the 
 portion close to the auricles is called the sinus venosus, and 
 the more distant the ductus venosus. We shall give to the 
 whole trunk the name used by the older observers, the 
 meatus venosus. 
 
 Small arteries to various parts of the body are now 
 being given off by the aorta and its branches. The capil- 
 
124 
 
 THE THIRD DAY. 
 FIG. 39. B. 
 
 [CHAP. 
 
 DIAGRAM OF THE VENOUS CIRCULATION ON THE THIRD DAT. 
 H. Heart. D. C. Ductus Cuvieri. S.V. Meatus venosus. Su. V. Superior 
 vertebral or anterior cardinal vein. C. Inferior or posterior cardinal vein. 
 Of. Omphalo-mesaraic vein. 
 
 laries in which these end are gathered into veins which 
 unite to form two main trunks on either side, the cardinal 
 veins, anterior and posterior (Fig. 23, Fig. 39 B, Su. V. and (7.), 
 which run parallel to the long axis of the body in the upper 
 part of the mesoblast, a little external to the protovertebrae. 
 These veins, which have not by the third day attained to any 
 great importance, unite opposite to the heart, on each side, 
 into a short common trunk at right angles to themselves. 
 The two short trunks thus formed, which bear the name of 
 Ductus Cuvieri (Fig. 23, Fig. 39 B, D.C.), running trans- 
 versely straight inwards towards the middle line fall into the 
 sinus venosus. 
 
 The blood-vessels in the body of the embryo take their origin from the 
 mesoblast exclusively, and their formation is probably precisely similar to that of 
 the vessels in the vascular area, branches being given out from or brought into 
 connection with the aorta and omphalo-mesaraic veins, in the same way as 
 branches were described as springing from or meeting the earliest formed vas- 
 cular channels. 
 
 His, carrying out the views we have already referred to, believed that 
 parablastic elements grew inwards along the omphalo-mesaraic trunks, through 
 the length of the heart and so onwards into the aorta and all its branches, until 
 
V-] 
 
 THE TAIL-FOLD. 
 
 125 
 
 the whole archiblastic framework of the embryo became riddled by a network 
 of parablast. From these parablastic elements he considered there arose not 
 only the epithelium (endothelium) of the blood-vessels and lymphatic spaces, 
 but also all the connective-tissue elements of the body, the archiblast being 
 represented in the vessels of the adult body by the muscular fibres alone. 
 
 17. As we stated above (p. 87), the folding in of the 
 splanchnopleure to form the alimentary canal is proceeding 
 with great rapidity, the tail-fold as well as the head -fold con- 
 tributing largely to the result. 
 
 The formation of the tail-fold is very similar to that of 
 the head-fold. At the extreme hind end of the embryo, at 
 the tip of the tail (Fig. 40, i), there is no cleavage of the 
 mesoblast, and therefore neither somatopleure nor splanchno- 
 pleure. The tail is a solid, somewhat curved, blunt cone of 
 mesoblast, immediately coated with the superficial epiblast 
 
 FIG. 40. 
 
 SECTION or THE TAIL-END OF AN EMBRYO (CHICK) OF THE THIRD DAT. (From 
 
 Dobrynin. ) 
 
 t. the tail. So. somatopleure, Spl. splanchnopleure. pp. pleuroperitoneal space. 
 The letters G and Dd are placed within the alimentary canal : a more 
 detailed explanation of the figure is given under Fig. 50. 
 
 except at the upper surface (corresponding to the back of the 
 embryo), where lies the pointed termination of the neural 
 tube. At some little distance forwards, towards the head, 
 the cleavage of the mesoblast begins, and the somatopleure 
 diverges from the splanchnopleure, the latter, as in the head- 
 fold, being folded in to a greater extent than the former. Ex- 
 cept for the absence of the cephalic enlargement of the neural 
 tube, the presence of distinct protovertebrae up to nearly the 
 
126 THE THIRD DAY. [CHAP. 
 
 FIG. 41. 
 
 SECTION THROUGH THE DORSAL REGION OF AN EMBRYO AT THE COMMENCEMENT 
 OF THE THIRD DAY. 
 
 M. C. medullary canal. Ck. notochord. p. v. protovertebra composed of an 
 
 investment of columnar cells enclosing a central nucleus of rounded cells. 
 
 w. d. Wolffian duct, which has commenced to travel down from the dorsal 
 
 surface of the mesoblast. A. o. dorsal aorta of right side. g. e. germinal 
 
 epithelium ; an epithelium of columnar cells lining the upper end of thepleuro- 
 
 peritoneal cavity, and which is related to the formation of Miiller's duct and 
 
 of the ovary. S. o. somatopleure. s. p. splanchnopleure. 
 
 The splanchnopleure is very little folded in, the embryonic sac being in this 
 
 middle region widely open to the yolk below. The somatopleure is much more 
 
 folded in. At a little distance outside the protovertebra is a ridge with 
 
 thickened mesoblast, the Wolffian ridge, marking the line along which the limbs 
 
 will be developed. Beyond this ridge the somatopleure suddenly descends to 
 
 form the body- wall (of the abdomen) ; it then ascends, and after a fold, probably 
 
 due to the action of the chromic acid> forms an angular projection at about the 
 
 level of the protovertebra. This projection is the lateral fold of the amnion, 
 
 the section having been taken just at that point between the head and tail where 
 
 the amnion is, on the third day, least developed. 
 
 Beyond the amniotic fold the somatopleure and splanchnopleure come into 
 apposition, arid still farther out completely coalesce. It will be observed that 
 the pleuroperitoneal space already reaches laterally far beyond the limits of 
 the embryo itself. 
 
 In the splanchnopleure is seen at V a section of a large branch of the 
 omphalo-mesaraic trunk. 
 
 The shading of the general mesoblast is diagrammatic. 
 
 The width of the cavity of the neural tube is unusually great. 
 
V.] THE MESENTERY. 127 
 
 actual termination of the tail, and certain features connected 
 with the development of the allantois, of which we shall 
 speak presently, the tail is a counterpart of the head. 
 
 So rapid is the closure of the splanchnopleure both in 
 front and behind, that two of the three parts into which 
 the digestive tract may be divided, are brought, on this day, 
 to the condition of complete tubes. 
 
 The first division, extending from the mouth to the 
 duodenum, is completely folded in by the end of the day ; so 
 likewise is the third division comprising the large intestine 
 and the cloaca. The middle division, corresponding to the 
 future small intestine, still remains quite open to the yolk- 
 sac below. 
 
 The attachment of the newly formed alimentary canal to 
 the body above is at first very broad, and only a thin stratum 
 of mesoblast separates the hypoblast of the canal from the 
 notochord and proto vertebrae ; even that may be absent under 
 the notochord (Fig. 41). During the third day, however, 
 along such portions of the canal as have become regularly 
 enclosed, i.e. the hinder division and in the posterior moiety 
 of the anterior division, the mesoblastic attachment becomes 
 narrower and (in a vertical direction) longer, the canal appear- 
 ing to be drawn more downwards (or according to the position 
 of the embryo forwards), away from the vertebral column. 
 
 In what may be regarded as the pleura! division of the 
 general pleuroperitoneal space, along that part of the ali- 
 mentary canal which will form the oesophagus, this with- 
 drawal is very slight (compare Fig. 32), but it is very marked 
 in the peritoneal space. Here such parts of the digestive 
 canal as are formed come to be suspended from the body 
 above by a narrow flattened band of mesoblastic tissue which 
 reaches from the neighbourhood of the notochord, and be- 
 comes continuous with the mesoblastic coating which wraps 
 round the hypoblast of the canal. This flattened band is the 
 mesentery, shewn commencing in Fig. 44, and much more 
 advanced in Fig. 47, M. It is covered on either side by a 
 layer of flat cells, while its interior is composed of indifferent 
 tissue. 
 
 The front division of the digestive tract consists of three 
 parts. The most anterior part, the oesophagus, still ends 
 blindly in front, and reaches back as far as the level of the 
 
128 
 
 THE THIRD DAY. 
 
 [CHAP 
 
 liind end of the heart. Its transverse section, which up to 
 the end of the second day was somewhat crescent-shaped, with 
 the convexity downwards, becomes on this day more nearly 
 circular. Close to its hinder limit, the lungs (Fig. 42, Ig), of 
 whose formation we shall speak directly, take their origin. 
 
 FIG. 42. 
 
 DIAGRAM OP A PORTION OF THE DIGESTIVE TRACT OP A CHICK UPON THE 
 FOURTH DAY. (Copied from Gotte.) 
 
 The black inner line represents the hypoblast, the outer shading the mesoblast. 
 Iff. lung-diverticulum with expanded termination, forming the primary 
 lung- vesicle. St. stomach. I. two hepatic diverticula with their terminations 
 united by solid rows of hypoblast cells, p. diverticulum of the pancreas 
 with the vesicular diverticula coming from it. 
 
 The portion of the digestive canal which succeeds the 
 oesophagus, becomes towards the close of the third day 
 somewhat dilated (Fig. 42, St) ; the region of the stomach is 
 thus indicated. 
 
 The hinder or pyloric end of the stomach is separated by 
 a very small interval from the point where the complete 
 closing in of the alimentary canal ceases, and where the 
 splanchnopleure-folds spread out over the yolk. This short 
 tract is nevertheless clearly marked out as the duodenum by 
 the fact that from it, as we shall presently point out, the 
 rudiments of ducts of the liver and pancreas are beginning to 
 be formed. It forms the third part of the front division. 
 
 The posterior division, corresponding to the great intestine 
 and cloaca, is from its very first formation nearly circular in 
 
V.] THE LUNGS. 129 
 
 section and of a larger bore than the oesophagus. Up to the 
 end of the day it is still completely closed at the hinder 
 extremity, which however is somewhat swollen to form the 
 cloaca. 
 
 18. The lungs are in origin essentially buds or processes 
 from the primitive oesophagus. 
 
 If the alimentary canal of the chick at the end of the 
 third day be dissected out and laid open, there will be seen 
 on either side of the hinder end of the oesophagus a short 
 pouch or diverticulum enveloped in a mass of mesoblast 
 (Fig. 42, Ig. ; here however they are somewhat advanced, 
 the specimen belonging to the fourth day). These pouches 
 are the early rudiments of the lungs. Their mode of origin 
 is as follows. 
 
 FIG. 43. 
 
 FOUK DIAGKAMS ILLUSTRATING THE FOEMATION OF THE LUNGS. 
 
 (Copied from Gotte.) 
 
 a. mesoblast. 1. hypoblast. d. cavity of digestive canal. I. cavity of the 
 
 pulmonary diverticulum. 
 
 In (i) the digestive canal has commenced to be constricted into an upper 
 and lower canal; the former the true alimentary canal, the latter the pulmonary 
 tube; the two tubes communicate with each other in the centre. 
 
 In (2) the lower (pulmonary) tube has become expanded. 
 
 In (3) the expanded portion of the tube has become constricted into two 
 tubes, still communicating with each other and with the digestive canal. 
 
 In (4) these are completely separated from each other and from the digestive 
 canal, and the mesoblast has also begun to exhibit externally changes corre- 
 sponding to the internal changes which have been going on. 
 
 E. 9 
 
130 THE THIRD DAY. [CHAP. 
 
 At a point immediately behind (or when the embryo is placed 
 on its face, above) the heart, the cavity of the alimentary canal 
 is compressed laterally, and at the same time constricted in 
 the middle so that its transverse section (Fig. 43, 1) is some- 
 what hour-glass-shaped, and shews an upper or dorsal cham- 
 ber d, joining on to a lower or ventral chamber I by a short 
 narrow neck. Th& lower chamber then becomes broader 
 than high (Fig. 43, 2) while its under wall is raised up in a 
 median fold, which partially divides the chamber into two 
 lateral parts (Fig. 43, 3). As a result of these folds, the 
 original simple tube becomes divided into three grooves or 
 incomplete tubes, whose cavities all communicate with each 
 other at the centre of the original canal : into a single tube 
 above, which is the true oesophagus, and into two tubes 
 below, each of which is a rudiment of a lung. The presence 
 of these three incomplete tubes may be traced, by sections 
 at various levels, for a certain distance along the alimentary 
 canal, and are then lost, the canal once more returning to 
 the condition of a simple tube. 
 
 The median fold or partition then rises up so as com- 
 pletely to shut off the three cavities from each other (Fig. 
 43, 4). The isolation commences behind and travels thence 
 forwards, but never quite reaches the point in front where 
 the division into three cavities begins. As a consequence 
 the lower pulmonary tubes, though closed behind, open into 
 the oesophagus in front. In other words, by the constriction 
 and median folds which we have described, the two short 
 pouches or diverticula of the lungs are developed from the 
 under (or anterior) surface of the oesophagus. 
 
 At their first origin both diverticula together with the 
 alimentary canal itself are invested in one common rounded 
 mass of mesoblast whose external contour bears no marks of 
 the internal changes which are going on. By and bye, as 
 the diverticula diverge behind from the median line, they 
 carry the mesoblast with them (Fig. 43, 4), Then for the 
 first time they become evident from the outside. 
 
 The subsequent history of these diverticula may with convenience be briefly 
 described here. 
 
 At first the two diverticula have separate openings into the oesophagus ; but 
 as the point at which they enter is carried further forwards (or rather as they in 
 the process of growth are carried backwards), they unite near their base to form 
 
V.] THE LIVER. 131 
 
 a common tube opening on the under surface of the oesophagus. This tube is 
 the trachea. 
 
 At the end of each of the primary diverticula is a small vesicle which may 
 be called the primary lung-vesicle. It appears ultimately to become the abdo- 
 minal air-sac. 
 
 The mesoblast round the primary diverticula becomes greatly thickened. 
 Into it secondary diverticula, still lined by hypoblast, penetrate; these again 
 give rise to tertiary branches and thus the bronchial tubes are formed. At right 
 angles'to the finest of these the arborescent branches so characteristic of the avian 
 lung are given off. The whole pulmonary structure is therefore the result of the 
 growth by budding of a system of branched hypoblastic tubes in the midst of a 
 mass of mesoblastic tissue, the hypoblastic elements giving rise to the epithelium 
 of the tubes and the mesoblast providing the elastic, muscular, cartilaginous, 
 connective and other tissues of the tracheal and bronchial walls. 
 
 The air-sacs are primarily the dilated ends of the primitive diverticula or of 
 their main branches. At first there are three air-sacs on each side : one abdo- 
 minal (the end of the primitive diverticulum), another thoracic, and a third 
 extra-thoracic. An additional thoracic, and an additional extra -thoracic, making 
 in all five air-sacs on each side, appear at a later period. 
 
 The pulmonary blood-vessels penetrate into the mesoblast surrounding the 
 bronchial tubes on about the iith day. 
 
 19. The liver is the first formed chylopoietic appendage 
 of the digestive canal, and arises between the 55th and 60th 
 hour as a couple of diverticula one from either side of the 
 duodenum immediately behind the stomach (Fig. 42, I.). 
 These diverticula, though composed of both hypoblast and 
 mesoblast, are, according to Gotte, in the first instance solid, 
 and only subsequently become hollow. The right one is, 
 in all cases, from the first longer, but of smaller diameter 
 than the left. Situated a little behind the heart, they 
 embrace between them the meatus venosus or united trunk 
 of the omphalo-mesaraic veins, which as it passes between 
 them exhibits numerous small bulgings. As yet the vein 
 arid the diverticula, though in close contact, are not connected. 
 
 Towards the end of the third day there may be observed 
 in the greatly thickened mesoblastic investment of either 
 diverticulum a number of cylindrical solid aggregations of 
 cells connected with, and apparently outgrowths from, the 
 hypoblast of the diverticulum. These cylinders rapidly 
 increase in number, apparently by division, their somewhat 
 swollen peripheral extremities come into contact and unite. 
 And thus, about the ninetieth hour, a sort of network of solid 
 thick strings of hypoblastic cells is formed, the mesoblast in 
 the meshes of the network becoming at the same time largely 
 converted into blood-vessels. In addition to this network of 
 solid hypoblastic cylinders, the diverticula also send out 
 
 92 
 
132 THE THIRD DAY. [CHAP. 
 
 hollow processes, lined with hypoblast. Each diverticulum 
 becomes in this way surrounded by a thick mass composed 
 partly of solid cylinders, and to a less extent of hollow 
 processes, continuous with the cylinders on the one hand, and 
 the main diverticulum on the other, all knit together with 
 commencing blood-vessels and unchanged mesoblastic tissue. 
 Between the two masses runs the meatus venosus, with the 
 bulgings on which, referred to above, the blood-vessels in 
 each mass are connected. 
 
 Early on the fourth day each mass sends out underneath 
 the meatus venosus a solid projection of hypoblastic cylinders 
 towards its fellow, that from the left side being much the 
 longest. The two projections unite and form a long solid 
 wedge, which passes obliquely down from the right (or from 
 the embryo lying on its left side, the upper) mass to the 
 left (or lower) one. In this new wedge may be seen the 
 same arrangement of a network of hypoblastic cylinders 
 filled in with vascular mesoblast as in the rest of the 
 liver. The two original diverticula with their investing 
 masses represent respectively the right and left lobes of the 
 liver, and the wedge-like bridge connecting them is the 
 middle lobe. 
 
 During the fourth and fifth days the growth of the solid, 
 lobed liver thus formed is very considerable ; the hypoblastic 
 cylinders multiply rapidly, and the network formed by them 
 becomes very close, the meshes containing little more than 
 blood-vessels. The hollow processes of the diverticula also 
 ramify widely, each branch being compo3ed of a lining of 
 hypoblast enveloped in a coating of spindle-shaped meso- 
 blastic cells. The blood-vessels are in direct connection 
 with the meatus venosus have become, in fact, branches of 
 it. It may soon be observed, that in those vessels which are 
 connected with the posterior part of the liver (Fig. 53), 
 the stream of blood is directed from the meatus venosus into 
 the network of the liver. In those connected with the anterior 
 part the reverse is the case ; here the blood flows from the 
 liver into the meatus venosus. The thick network of solid 
 cylinders represents the hepatic parenchyma of the adult 
 liver, while the hollow processes of the diverticula are the 
 rudiments of the biliary ducts. 
 
 The exact morphological significance of these anastomosing cylinders, and 
 
V.] 
 
 THE PANCREAS. 133 
 
 the manner of their ultimate metamorphosis into the ordinary hepatic tissue, is 
 not as yet quite clear. If we suppose each solid cylinder to represent a duct 
 with its lumen almost, but not quite, completely obliterated, we should gain a 
 view agreeing very closely with that put forward by Hering on the structure 
 of the adult liver. 
 
 During the fifth day, a special sac or pouch is developed 
 from the right primary diverticulum. This pouch, consisting 
 of an inner coat of hypoblast, and an outer of mesoblast, is 
 the rudiment of the gall-bladder. 
 
 20. About the middle of the third day, the pancreas 
 (Fig. 42, p.) also appears, but its exact mode of origin is still 
 somewhat doubtful. 
 
 According to G-otte (Beit. z. entwicU. des Darmcanals des Huhnchens) it 
 commences as a thickening of both the hypoblast and mesoblast of a portion of 
 the wall of the digestive canal on the same level as the left diverticulum of 
 the liver. The hypoblast in the centre of this thickening becomes hollow, 
 forming a cavity connected with the inside of the digestive canal by a narrow 
 opening. Around this cavity processes of hypoblast are seen on the fourth day 
 stretching into the surrounding mesoblast. These processes, which are at first 
 solid but afterwards become hollow and ultimately branched, are in the early 
 stages so completely covered up by mesoblast that they are not visible on the 
 exterior. The primary cavity elongates into the duct, the hollow processes 
 representing its branches* On the sixth day a new similar outgrowth takes 
 place between the primary one and the stomach. This, which ultimately 
 coalesces with its predecessor, gives rise to the second duct, and forms a 
 considerable part of the adult pancreas. A third duct is formed at a much 
 later period. According to this view, with which those put forward by Kolliker 
 and Remak in the main agree, the so-called 'secreting' cells of the pancreas 
 as well as the epithelial lining of the ducts are derived from hypoblast. 
 Schenk (Die Bauchspetiheldriise des Embryos. Anat. u. Physiol. Untersuch- 
 ung. Wien. S. I.) however is of opinion that the former originate in a 
 transformation of the mesoblast, the hypoblast giving rise to the epithelium 
 of the ducts only. 
 
 Shortly after the first appearance of the pancreas, the 
 spleen appears as a thickening of the mesentery of the stomach 
 (mesogastricum) and is therefore entirely a mesoblastic struc- 
 ture. 
 
 Its development has been recently investigated by Peremeschko (Sitz. der k. 
 Akad. in .Wien, Bd. 56, 1867) and by W. Miiller (Strieker's Histology). 
 
 According to these investigators, the mass of mesoblast which forms the 
 spleen becomes early separated by a groove on the one side from the pancreas and 
 on the other from the mesentery. Some of its cells become elongated, and send 
 out processes which, uniting with like processes from other cells, form the trabe- 
 cular system. From the remainder of the tissue are derived the cells of the spleen 
 pulp, which frequently contain more than one nucleus. Especial accumulations 
 of these take place at a later period to form the so-called Malpighian corpuscles 
 of the spleen. 
 
 21. The thyroid body is also formed towards the end of 
 the third day, in connection with the alimentary canal. 
 
134 THE THIRD DAY. [CHAP. 
 
 According to Miiller (Ueber die Entwickelung der ScTiilddri'ise. Jenaische 
 Zeitschrift, 1871) who has recently studied its development with great care, the 
 thyroid body arises on the third day as an, involution from the hypoblast of the 
 throat opposite the point of origin of the second arterial arch. This involution 
 becomes by the fourth day a solid mass of cells, and by the fifth ceases to be 
 connected with the epithelium of the throat, becoming at the same time bilobed. 
 By the seventh day it has travelled somewhat backwards, and the two lobes 
 have completely separated from each other. By the ninth day the whole is 
 invested by a capsule of connective tissue, which sends in septa dividing it into 
 a number of lobes or solid masses of cells, and by the sixteenth day it is a paired 
 body composed of a number of follicles, each with a ' membrana propria,' and 
 separated from each other by septa of connective tissue, much as in the adult. 
 
 22. Coincidently with the appearance of these several 
 rudiments of important organs in the more or less modified 
 splanchnopleure-folds, the solid trunk of the embryo is 
 undergoing marked changes. 
 
 When we compare a transverse section taken through 
 say the middle of the trunk at the end of the third day 
 (Fig. 44), with a similar one of the second day (Fig. 20), or 
 even the commencement of the third day (Fig. 41), we 
 are struck with the great increase of depth (from dorsal 
 to ventral surface) in proportion to breadth. This is partly 
 due to the slope of the side walls of the body having become 
 much steeper as a direct result of the rapidly progressing 
 folding off of the embryo from the yolk-sac. But it is also 
 brought about by the great changes both of shape and 
 structure which are taking place in the protovertebrae as 
 well as by the development of a mass of tissue between the 
 notochord and the hypoblast of the alimentary canal. 
 
 23. The protovertebra3 on the second day, as seen in a 
 transverse section, Fig. 20, P.v., are somewhat quadrilateral 
 in form but broader than high. Each at that time consists 
 of a somewhat thick cortex of radiating rather granular 
 columnar cells, enclosing a small kernel of spherical more 
 transparent cells. 
 
 Remak and after him Kolliker have described the centre of the proto- 
 vertebrsfi as being simply fluid without structural elements. His explicitly 
 denies this in the case of the protovertebrse of the neck, and it seems probable 
 that the centre is in all cases really occupied by transparent spherical cells. 
 
 Towards the end of the second and the beginning of 
 the third day, the central cells increase rapidly in number 
 (Fig. 41), and towards the end of the latter day (Fig. 44), as 
 it were lift up and push out the columnar cortex above and 
 at the outer side. In this way the portions forming the 
 
THE MUSCLE-PLATES. 
 
 135 
 
 SO 
 
 SECTION THROUGH THE DORSAL KEGION OP AN EMBRYO AT THE END OF THE 
 
 THIRD DAY. 
 
 Am. amnion. m. p. muscle-plate. C. F. cardinal vein. Ao. dorsal aorta. The 
 section passes through the point which the dorsal aorta is just commencing 
 to divide into two branches. Ck. notochord. W. d. Wolffian duct. W. b. 
 commencing differentiation of the mesoblast cells to form the Wolffian 
 body. ep. epiblast. SO. somatopleure. Sp. splanchnopleure. hy. hypoblast. 
 The section passes through the point where the digestive canal communicates 
 with the yolk-sac, and is consequently still open below. 
 
 This section should be compared with the section through the dorsal region of 
 an embryo at the commencement of the third day (Fig. 41). The chief differences 
 between them arise from the great increase in the space (now filled with meso- 
 blast-cells) between the notochord and the hypoblast. In addition to this we 
 have in the later section the completely formed amnion, the separation of the 
 muscle-plate from the protovertebrse, the formation of the Wolffian body, etc. 
 
 The mesoblast including the Wolffian body and the muscle-plate (m.p.) is 
 represented in a purely diagrammatic manner. The amnion, of which only the 
 inner limb or true amnion is represented in the figure, is seen to be composed of 
 epiblast and a layer of mesoblast ; though in contact with the body above the top 
 of the medullary canal, it does not in any way coalesce with it, as might be con- 
 cluded from the figure. 
 
136 THE THIRD DAY. [CHAP. 
 
 upper and outer border of the protovertebra become separated 
 from the rest of the cortex, the columnar cells of the latter 
 at the same time losiDg their distinctive characters and 
 ceasing to be distinguishable from the central cells. As a 
 consequence of this the whole protovertebra, while thus 
 increasing in breadth out of proportion to its height, becomes 
 split up into two portions which lie one above the other. 
 Of these the upper one, which is from the first the most 
 flattened and longest, follows the curvature of the body-wall 
 and thus from being nearly horizontal comes to slope at a 
 considerable angle. It now receives the name of muscle-plate, 
 Fig. 44, m.p. Of its subsequent changes we shall have to speak 
 in a succeeding chapter. 
 
 The remaining portion of the original protovertebra is 
 still called protovertebra and begins to extend inwards over 
 the neural canal above and towards the notochord below. 
 
 24. Meanwhile the breadth or rather depth of the trunk 
 is also being increased by the development of mesoblastic 
 cells between the notochord and hypoblast. 
 
 In a transverse section of a 45 hours' embryo a consider- 
 able mass of cells may be seen collected between the pro- 
 tovertebra and the point where the divergence into somato- 
 pleure and splanchnopleure begins (Fig. 20 just below W.d., 
 also Fig. 41, the diagrammatically shaded part lying between 
 p.v. and g.e). This mass of cells, which we may speak of as 
 the intermediate cell-mass, now passes without any very sharp 
 line of demarcation into the protovertebra itself ; and as the 
 folding in of the side wall progresses, increases in size and 
 grows in between the notochord and the hypoblast, but does 
 not accumulate to a sufficient extent to separate them 
 widely until the end of the third or beginning of the fourth 
 day. 
 
 The fusion between the intermediate cell-mass and the 
 outer and under portions of the altered pro to vertebras 
 becomes so complete on the third day that it is almost 
 impossible to say which of the cells in the immediate 
 neighbourhood of the notochord are derived from the proto- 
 vertebrse and which from the intermediate cell-mass. It 
 seems probable however that the cells which form the 
 immediate investment of the notochord really belong to the 
 protovertebraB. 
 
V.] THE CRANIAL NERVES. 137 
 
 Schenk (Wien. Sitz. Bericlit. 1868) describes all the cells which invest 
 the hypo-blast of the digestive tract, as primarily derived from the proto- 
 vertebrse, with the exception of the peritoneal epithelium, which alone, he con- 
 siders, is the representative of the original mesoblast of the splanchnopleure. 
 According to this view, the muscles of the walls of the alimentary canal, and the 
 'hypaxial' muscles, are derived from the original protovertebrse, quite as much 
 as those muscles which spring out of the muscle-plate. In the absence of any 
 satisfactory means of distinguishing the cells of the intermediate cell-mass 
 from those of the protovertebree, this view must be considered as at least very 
 doubtful. 
 
 25. In the mesoblast, which lies by the side of the 
 hind brain, and which though not divided into protover- 
 tebrse is the prolongation forwards of the same column of 
 mesoblast out of which in the trunk the protovertebrse are 
 formed, there appear on either side in the course of the third 
 day a series of four small opaque masses, somewhat pearshaped 
 with the stalk directed away from the middle line. These 
 are the rudiments of four cranial nerves, of which two lie in 
 front of and two behind the auditory vesicle. 
 
 The most anterior of these is the rudiment of the fifth 
 nerve (Figs. 25, V. 45, V). Its narrowed outer portion or 
 
 HEAD OF AN EMBRYO CHICK OF THE THIRD DAY (seventy-five hours) VIEWED 
 
 SIDEWAYS AS A TRANSPARENT OBJECT (from Huxley). 
 
 la. cerebral hemispheres. 16. vesicle of the third ventricle. II. mid-brain. 
 
 III. hind-brain, g. nasal pit. a. optic vesicle. &. otic vesicle, d. infundi- 
 
 bulum. e. pineal body. h. notochord. V. fifth nerve. VII. seventh nerve. 
 
 VIII. united glossopharyngeal and pneumogastric nerves, i, 2,^3, 4, 5 the 
 
 five visceral folds. 
 
 The stage here represented is a little later than that shewn in Fig. 25, with 
 which it should be compared. 
 
138 THE THIRD DAY. [CHAP. 
 
 stalk divides into two bands or nerves. Of these one passing 
 towards the eye terminates at present in the immediate 
 neighbourhood of that organ. Compare Fig. 46. The other 
 branch (the rudiment of the inferior maxillary branch of the 
 fifth nerve) is distributed to the first visceral arch (Fig. 46). 
 
 The second mass (Fig. 25, VII. 45, VII.) is the rudiment 
 of the seventh, or facial nerve. It is the nerve of the second 
 visceral arch. 
 
 The two masses behind the auditory vesicle represent 
 the glossopharyngeal and pneumogastric nerves (Fig. 45, VIII., 
 Fig. 46, G. Ph. and Pg.}. At first united, they subsequently 
 become separate. The glossopharyngeal supplies the third 
 arch, and the pneumogastric the fourth arch. 
 
 These four masses, representing four important mixed cranial nerves, 
 seem to be derived directly from the mesoblast surrounding the hind-brain. 
 It is worthy of notice that they are mixed, sensory and motor, nerves; for, 
 restricted as are the sensory functions of the seventh, and the motor func- 
 tions of the pneum.ogastric in the adult mammal, the study of their com- 
 parative physiology leaves no doubt as to the essentially mixed nature of each. 
 It is also worthy of note that of the third, fourth and sixth nerves, no such early 
 rudiments appear ; and there are reasons for thinking that these are in reality 
 intercranial branches, the third and fourth of the fifth, and the sixth of the 
 seventh nerves. The purely sensory nerve or rather sense-nerve, the auditory, 
 seems to have a different origin altogether from all the above, though it may 
 perhaps be looked upon as the dorsal branch of the seventh, while the erratic 
 hypoglossal appears to be distinctly a spinal nerve. 
 
 Of the interesting relations of these cranial nerves to the visceral arches, we 
 shall have to speak more fully in the second part of this work, when we describe 
 the more primitive forms in the lower vertebrata. 
 
 At the same time that these ganglia make their appearance, or a little earlier, 
 near the beginning of the third or end of the second day, there may be seen in 
 the region of the hind-brain, lines which appear to divide off the mesoblast on 
 either side into masses somewhat resembling protovertebree. Of these masses 
 there are four or five on each side, generally three in front of. and two behind 
 the optic vesicle. They were first noticed by Remak, and are easily dis- 
 tinguished from the rudiments of the cranial nerves. They at first sight suggest 
 the idea of an initial and transitory segmentation of the cranial mesoblast into 
 protovertebrse. It seems possible that they are, in reality, appearances pro- 
 duced by a series of very characteristic transverse wrinkles into which the walls 
 of the hind-brain are at this time thrown, arid which subsequently disap- 
 pearing altogether, as the walls increase in thickness, may perhaps be viewed 
 as indications of an aborted segmentation of the hind-brain into a series of 
 vesicles. The true nature of these quadrate masses is still very problematical. 
 
 26. On the second day the newly formed Wolffian duct 
 extended along the greater part of the length of the embryo 
 as a tube resting on the mass of cells which we have already 
 called the intermediate cell-mass. 
 
V.] THE WOLFFIAN DUCT. 139 
 
 On the third day, in consequence of the continually 
 folding in of the somatopleure and especially of the splanch- 
 nopleure, as well as owing to the changes taking place in the 
 protovertebrao, the Wolffian duct undergoes a remarkable 
 change of position. Instead of lying as on the second day 
 immediately under the epiblast (Fig. 20, W.d.), it is soon 
 found to have apparently descended into the middle of the 
 intermediate cell-mass (Fig. 41, w.d.) and at the end of 
 the third day occupies a still lower position and even pro- 
 jects somewhat into the pleuroperitoneal cavity. (Fig. 44, 
 W.d.) 
 
 Towards the end of the day the rudiments of the Wolffian 
 bodies (Fig. 44, W.b.) begin to make their appearance in con- 
 nection with the ducts, but the consideration of these may 
 conveniently be reserved to the next chapter. 
 
 27. The chief events then which take place on the 
 third day are as follows : 
 
 1. The turning over of the embryo so that it now 
 lies on its left side. 
 
 2. The cranial flexure round the anterior extremity of 
 the notochord. 
 
 3. The completion of the circulation of the yolk-sac ; 
 the increased curvature of the heart, and the demarcation of 
 its several parts ; the appearance of new aortic arches, and 
 of the cardinal veins. 
 
 4. The formation of four visceral clefts and five visceral 
 arches. 
 
 5. The involution to form the lens, and the formation 
 of the secondary optic vesicle. 
 
 6. The closing in of the otic vesicle. 
 
 7. The formation of the nasal pits. 
 
 8. The appearance of the vesicles of the cerebral hemi- 
 spheres ; the separation of hind-brain into cerebellum and 
 medulla oblongata. 
 
 9. The completion of the fore-gut and of the hind-gut ; 
 the division of the former into oesophagus, stomach and 
 duodenum, of the latter into large intestine and cloaca. 
 
 10. The formation of the lungs as two diverticula from 
 the alimentary canal immediately in front of the stomach. 
 
 11. The formation of the liver and pancreas: the former 
 as two diverticula from the duodenum, which subsequently 
 
140 THE THIRD DAY. [CHAP. V. 
 
 become united by solid outgrowths ; the latter as a single 
 diverticulum also from the duodenum. 
 
 12. The changes in the proto vertebrae and the appear- 
 ance of the muscle-plates. 
 
 13. The appearance of the cranial nerves in the meso- 
 blast adjoining the hind brain. 
 
 14. The change in position of the Wolffian duct. 
 
CHAPTER VI. 
 
 THE CHANGES WHICH TAKE PLACE DURING THE FOURTH 
 
 DAY. 
 
 1. ON opening an egg in the middle or towards the end 
 of the fourth day, a number of points in which progress has 
 been made since the third day are at once apparent. In the 
 first place, the general growth of the embryo has been very 
 rapid, so that its size is very much greater than on the 
 previous day. In the second place, the white of the egg has 
 still further diminished, the embryo lying almost in immediate 
 contact with the shell membrane. 
 
 The germinal membrane embraces more than half the 
 yolk, and the vascular area is about as large as a halfpenny. 
 
 Corresponding to the increased size of the embryo, there 
 is a great increase in the quantity of blood circulating in the 
 vascular area as a whole, though the sinus terminalis is 
 already less distinct 'than it was. 
 
 2. The amnion becomes increasingly conspicuous. It is 
 now seen as a distinct covering obscuring to a certain extent 
 the view of the body of the chick beneath, and all traces of 
 the junction of its folds are by this time lost. As yet there 
 is very little fluid in the amniotic sac proper, so that the 
 true amnion lies close upon the embryo. 
 
 3. The folding off of the embryo from the yolk sac has 
 made great progress. The splanchnic stalk, which on the 
 third day was still tolerably wide, inasmuch as about one 
 third of the total length of the alimentary canal was as yet 
 quite open to the yolk sac below, now becomes so much con- 
 stricted by the progressive closing in of the splanchnopleure 
 
142 
 
 THE FOURTH DAY. 
 
 [CHAP. 
 
 FIG. 46. 
 vir *^ 
 
 *. s ;\ *rlr?* c * 
 
 M.P 
 
 EMBRYO AT THE END OF THE FOURTH DAY SEEN AS A TRANSPARENT OBJECT. 
 
 The amnion has been completely removed, the cut end of the somatic stalk 
 is shewn at S.S. with the allantois (Al.) protruding from it. 
 
 C.H. cerebral hemisphere. F. B. fore brain or vesicle of the third ventricle 
 with the pineal gland (Pn.) projecting from its summit. M.B. mid brain. 
 Cb. cerebellum. IV. V. fourth ventricle. L. lens. ch.s. choroid slit. 
 Owing . to the growth of the optic cup the two layers of which it is 
 composed cannot any longer be seen from the surface, but the posterior 
 surface of the choroid layer alone is visible. Cen. V. auditory vesicle. 
 s.m. superior maxillary process. ijP. iF. etc. first, second, third and 
 fourth visceral folds. V. fifth nerve sending one branch to the eye, the 
 ophthalmic branch, and another to the first visceral arch. VII. seventh 
 nerve passing to the second visceral arch. G. Ph. glossopharyngeal nerve 
 passing towards the third visceral arch. Pg. pneumogastric nerve passing 
 towards the fourth visceral arch. iv. investing mass. No attempt has 
 been made in the figure to indicate the position of the dorsal wall of 
 the throat, which cannot be easily made out in the living embryo. 
 ch. notochord. The front end of this cannot be seen in the living embryo. 
 It does not end however as shewn in the figure, but takes a sudden bend 
 downwards and then terminates in a point. Ht. heart seen through the 
 walls of the chest. M. P. muscle-plates. W. wing. H. L. hind limb. 
 Beneath the hind limb is seen the curved tail. 
 
VI.] THE LIMBS. 143 
 
 folds, that the alimentary canal maybe said to be connected 
 with the yolk sac by a very narrow neck only. This rem- 
 nant of the splanchnic stalk we may now call the umbilical 
 duct; though narrow, it is as yet quite open, affording 
 still free communication between the inside of the yolk sac 
 and the interior of the alimentary canal. 
 
 The somatic stalk, though narrowing somewhat, is much 
 wider than the splanchnic stalk, so that a considerable 
 ring-shaped space exists between the two. 
 
 4. Another very prominent feature is the increase in the 
 cranial flexure. During the third day, the axis of the front 
 part of the head was about at right angles to the long axis of 
 the body ; the whole embryo being still somewhat retort- 
 shaped. On this day, however, the flexure has so much 
 increased that the angle between the long axis of the body 
 and that of the front segment of the head is an acute one, 
 and the mouth is turned so as completely to face the chest. 
 
 The tail-fold, which commenced to be noticeable during 
 the third day, has during this day increased very much, and 
 the somewhat curved tail (Fig. 46) forms quite a conspicuous 
 feature of the embryo. The general curvature of the body has 
 also gone on increasing, and as the result of these various 
 flexures, the embryo has very much the appearance of being 
 curled up on itself (Fig. 46). 
 
 5. The distinct appearance of the limbs must be reckoned 
 as one of the most important events of the fourth day. 
 
 Owing to the continued greater increase of depth than of 
 breadth, the body of the embryo appears in section (Fig. 47) 
 higher and relatively narrower than even on the third day, and 
 the muscle-plates, instead of simply slanting downwards, come 
 to be nearly vertical in position. Not far from the line 
 which marks their lower ends, the somatopleure, almost 
 immediately after it diverges from the splanchnopleure, is 
 raised up (Fig. 47, W.R.} into a low rounded ridge which runs 
 along nearly the whole length of the embryo from the neck 
 to the tail. 
 
 It is on this ridge, which is known as the Wolffian ridge, 
 tha,t the limbs first appear as flattened conical buds project- 
 ing outwards. They seem to be local developments of the 
 ridge, the rest of which becomes less and less prominent as 
 they increase in size. Each bud, roughly triangular in sec- 
 
144 
 
 THE FOUKTH DAY. 
 
 FIG. 47. 
 
 [CHAP. 
 
 nc. 
 
 SECTION THROUGH THE LUMBAR KEGION or AN EMBRYO AT THE END OF THE 
 FOURTH DAT. , . 
 
 n. c. neural canal, p. r. posterior root of spinal nerve with ganglion, a. r. anterior 
 root of spinal nerve. A. 6r. C. anterior grey column of spinal cord. A. W. C. 
 anterior white column of spinal cord just commencing to be formed, and 
 not very distinctly marked in the figure, m. p. muscle plate, ch. notochord. 
 W. R. Wolffian ridge. A. 0. dorsal aorta. V. c. a. posterior cardinal vein. 
 W. d. Wolffian duct; its section is not circular, owing to its being cut 
 through at a point where it is being joined by one of the tubules. W. b. 
 Wolffian body, consisting of tubules and Malpighian corpuscles. One of 
 the latter is represented on each side, that on the left hand having its 
 glomerulus entirely filled with blood -corpuscles, g. e. germinal epithelium. 
 M. d. commencing involution of germinal epithelium to form the duct of 
 Muller. d. alimentary canal. M . commencing mesentery. S. 0. somato- 
 pleure. S. P. splanchnopleure. V. blood-vessels, pp. pleuroperitoneal 
 cavity. 
 
VI,] THE NASAL PITS. 145 
 
 tion, consists of somewhat dense mesoblast covered by epiblast 
 which on the summit is thickened into a sort of cap. The 
 front limbs or wings (Fig. 46) arise just behind the level of 
 the heart, and the hind limbs in the immediate vicinity 
 of the tail. The first traces of them can be seen towards 
 the end of the third, but they do not become conspicuous 
 till the fourth day, by the end of which the two pairs may 
 be already distinguished by their different shapes. The front 
 limbs are the narrowest and longest, the hind ones being 
 comparatively short and broad. Both are flattened from 
 above downwards and become more so as their growth 
 continues. 
 
 6. In the head, the vesicles of the cerebral hemispheres 
 are rapidly increasing in size, overlapping the insignificant 
 olfactory vesicles in front, and encroaching on the 'tween- 
 brain or vesicle of the third ventricle behind. The mid-brain 
 is now, relatively to the other parts of the brain, larger than 
 at any pther epoch, and an indistinct median furrow on its 
 upper surface indicates its division into two lateral halves. 
 The great increase of the mesoblastic contents of the second- 
 ary optic vesicle or involuted retinal cup causes the two eye- 
 balls to project largely from the sides of the head (Fig. 48, Op). 
 The mass of mesoblast which invests all the various parts of the 
 brain, is not only growing rapidly below and at the sides, but 
 is also undergoing developments which result in the forma- 
 tion of the primitive skull, and of which we shall speak in 
 detail in a subsequent separate chapter. All these events, 
 added to the cranial flexure spoken of above, give to the 
 anterior extremity of the embryo a shape which it becomes 
 more arid more easy to recognize as that of a head. 
 
 7. Meanwhile the face is also being changed. The two 
 nasal pits were on the third day shallow depressions with 
 thickened borders complete all round. As the pits deepen on 
 the fourth day by the growth upwards of their rims, a break 
 is observed in each rirn in the form of a groove (Fig. 48, N) 
 directed obliquely downwards towards the cavity of the mouth. 
 The fronto-nasal process or median ridge (Fig. 48, nf), which 
 on the third day rose up between the superficial projections 
 caused by the bulging anterior extremities of the vesicles of 
 the cerebral hemispheres, and on the fourth day becomes 
 increasingly prominent, separates the two grooves from each 
 
146 THE FOUETH DAY. [CHAP. 
 
 FIG. 48. 
 
 A. HEAD OF AN EMBRYO CHTCK OF THE FOURTH DAT VIEWED FROM BELOW 
 
 AS AN OPAQUE OBJECT. (Chromic acid preparation). 
 
 CH. cerebral hemispheres. FB. vesicle of the third ventricle. Op. eyeball. 
 nf. naso-frontal process. M. cavity of mouth. S. M. superior maxillary 
 process of .P. i, the first visceral fold (inferior maxillary process). F. 2, F. 3. 
 second and third visceral folds. N. nasal pit. 
 
 In order to gain the view here given the neck was cut across between the 
 third and fourth visceral iolds. In the section e thus made, are seen the 
 alimentary canal al with its collapsed walls, the neural canal n.c., the notochord 
 ch., the dorsal aorta AO., and the vertebral veins V. 
 
 The incision has been carried just below the upper limit of the pleuroperi- 
 toneal cavity, consequently a portion of the somatopleure appears at the angle 
 between the two third visceral folds. Almost embraced by the piece of somato- 
 pleure is seen the end of the bulbus arteriosus Ao. 
 
 In the drawing the nasal groove has been rather exaggerated in its upper 
 part. On the other hand the lower part of the groove where it runs between 
 the superior maxillary process S.M. and the broad naso-frontal process was, in 
 this particular embryo, extremely shallow and indeed hardly visible. Hence 
 the end of the superior maxillary process seems to join the inner and not, as 
 described in the text, the outer margin of the nasal groove. A few hours later 
 the separation of the two would have been very visible. 
 B. The same seen sideways, to shew the visceral folds. Letters as before. 
 
 other, and helps to form the inner wall of each of them. 
 Abutting on the outer side of each groove and so helping to 
 form the outer wall of each, lie the ends of the superior 
 maxillary processes of the first visceral arch (Fig. 48 B, sm), 
 which like the fronto-nasal process are increasing in size. By 
 their continued growth, the groove is more and more deepened, 
 and leading as it does from the nasal pit to the cavity of the 
 mouth, may already be recognized as the rudiment of the 
 passage of the posterior nares. 
 
VI.] THE CRANIAL NERVES. 147 
 
 8. Daring the latter half of the fourth day there appears 
 at the bottom of the deep lozenge-shaped cavity of the mouth, 
 in the now thin wall dividing it from the alimentary canal, a 
 longitudinal, or according to Gotte a vertical slit which, soon 
 becoming a wide opening, places the two cavities in complete 
 communication. 
 
 The cavity of the mouth, being, it will be remembered, 
 formed partly by depression, partly by th,e growth of the sur- 
 rounding folds, is lined entirely with epiblast, from which the 
 epithelium of its surface and of its various glands is derived. 
 In this respect, as Remak pointed out, it differs from all the 
 rest of the alimentary canal, w^hose whole epithelium is formed 
 out of hypoblast. 
 
 9. By the side of the hind-brain, in which the cerebellum, 
 through the increased thickening of its upper walls, is be- 
 coming more and more distinct from the medulla oblongata, 
 both in front and behind the auditory vesicle, in which the 
 rudiments of the cochlea and recessus vestibuli are already 
 visible, the cranial ganglia and nerves are acquiring increased 
 distinctness and size. They may be very plainly seen when 
 the head of the fresh embryo is subjected to pressure. 
 
 The foremost is the fifth cranial nerve (Fig. 46, V.) with 
 its Gasserian ganglion ; it lies a little way behind the anterior 
 extremity of the notochord immediately below the cerebellum. 
 Next to this comes the seventh (Fig. 46, VII.} nerve, starting 
 just in front of the ear-vesicle, and extending far downwards 
 towards the second visceral arch. The two nerves which lie 
 behind the ear- vesicle are now obviously separate from each 
 other; the front one is the glossopharyngeal (Fig. 46, G. Ph.], 
 and the hinder one already shews itself to be the pneumo- 
 gastric (Fig. 46> P#.)- 
 
 10. Besides the progressive changes of the alimentary 
 canal and its surroundings, which we incidentally described 
 in the last chapter, and the closure of the mid-gut to form the 
 umbilical duct, of which we have also already spoken, a totally 
 new and most important appendage of the digestive tract, 
 the allantois, becomes for the first time conspicuous on this 
 day, though the first rudiments of it appeared on the third. 
 
 Soon after its appearance the allantois may easily be recog- 
 nized as a pear-shaped vesicle lying in the hinder district of 
 the pleuroperitoneal space, and connected with the under 
 
148 THE FOURTH DAY. [CHAP. 
 
 surface of the cloaca by a long hollow stalk, which places its 
 cavity in communication with that of the alimentary canal. 
 Both vesicle and stalk have an outer coat of mesoblast and 
 an inner lining of what apparently is hypoblast. So much 
 any observer may readily determine for himself; but of the 
 earliest stages of the development of this organ different 
 embryologists have given very different accounts. 
 
 Von Baer believed that, soon after the cloaca was formed by the enlargement 
 of the caecal hind end of the alimentary canal, the allantois arose from it as 
 a spherical diverticulum, generally visible about the middle of the third day, in 
 whose formation both of the coats of the alimentary canal took part. This 
 spherical diverticulum gradually lengthened out into a pear-shaped vesicle, 
 connected with the cloaca by a hollow stalk which rapidly narrowed and 
 lengthened, until the allantois formed an independent hollow body, composed 
 of an outer coat of mesoblast and a lining of hypoblast, and communicating 
 with the cloaca by a narrow tube of the same construction. 
 
 Keichert (Entwicklungsgeschichte, s. 186) on the other hand stated that the 
 allantois was formed of two solid outgrowths from the mesoblast of the somato- 
 pleure, which subsequently coalesced and became hollow; but believed that it 
 was primarily connected with the Wolffian ducts and not with the cloaca. 
 
 According to Remak (Entwicklung, 57, 58) it is formed by two solid 
 vascular outgrowths of the mesoblast of the body-wall, one on each side of the 
 middle line, which project in the pleuroperitoneal cavity near to the cloaca. 
 These two outgrowths coalesce, and then grow up, till they come in contact with 
 
 FIG. 49. 
 
 LONGITUDINAL SECTION OF THE TAIL-END OP AN EMBRYO CHICK AT THE 
 
 COMMENCEMENT OF THE THIRD DAY (Dobrjnin). 
 
 t. the tail, m. the axial mesoblast of the body, about to form the protovertebras. 
 x f . the roof of x". the neural canal. Dd. the hind end of the hind-gut. SO. 
 somatopleure. Spl. splanchnopleure. u. the mesoblast of the splancbno- 
 pleure carrying the vessels of the yolk-sac, pp. pleuroperitoneal cavity. 
 Df. the epithelium lining the pleuroperitoneal cavity. All. the commencing 
 allantois. w. and y. the hypoblast thickened and projecting on either side of 
 the opening of the allantois. 
 
VL] 
 
 THE ALLANTOIS. 149 
 
 the wall of the cloaca: with this they unite, and form together a solid 
 spherical body, bearing on its external surface a median furrow, indicating its 
 double origin. A narrow diverticulum of hypoblast now passes into the mass, 
 and forms within it a cavity, which is at first small and, corresponding to the ex- 
 ternal contour of the body, to a certain extent double. The hypoblast diverticulum 
 grows rapidly, while its mesoblastic covering remains nearly stationary, so that 
 the mesoblast finally comes to form a thin coating only over the hypoblast. 
 
 His (op. cit. ) gives a somewhat elaborate and complicated account of the 
 development of the allantois; which is accepted by Waldeyer (Eierstock und 
 Ei) and Bornhaupt (Untersuckuny iiber die Entwickelung des Urino-genitalsy stems 
 beim Hiihnchen, Riga, 1867). 
 
 It appears to be nearly the same as the fuller account given by Dobrynin 
 (Ueber die erste Anlage der Allantois. Sitz. der k. Akad. Wien, Bd. 64, 1871), 
 of which the following is an abstract. 
 
 When the first commencement of the hind fold takes place, immediately 
 beyond the point where the hypoblast turns back to assume its normal direction 
 over the yolk-sac, a narrow diverticulum which points backwards and some- 
 what upwards is formed by a special flexure of the splanchnopleure. The open 
 end of the diverticulum, Fig. 49, All. f looks forwards towards the wide opening 
 connecting the digestive tract with the yolk-sac; its blind end points directly 
 towards the pleuroperitoneal cavity. This diverticulum is the commencing 
 allantois. It is lined by hypoblast, while its exterior is composed of the mesoblast 
 of the splanchnopleure. 
 
 As the folding in to form the digestive tract increases, the diverticulum alters 
 
 FIG. 50* 
 
 LONGITUDINAL SECTION OP THE TAIL-END OF AN EMBRYO CHICK AT THE 
 MIDDLE OF THE THIRD DAY (Dobrynin). 
 
 t. the tail; the line of reference points to the axial mesoblast at the tail. 
 x. epiblast. SO. somatopleure. ra. mesoblast to form the body wall. 
 V. commencing amniotic fold. Hp. space between the true and the false 
 amnion. pp. Pleuroperitoneal cavity. Spl. splanchnopleure. G. Cloacal 
 enlargement of the alimentary canal. Dd. dorsal wall of the alimentary 
 canal. All. vesicle of the allantois having a wide opening into the alimen- 
 tary canal. 
 
150 THE FOURTH DAY. [CHAP. 
 
 its position and becomes quite parallel with the commencing digestive tract. 
 Its cavity is separated from that of the digestive canal by a projection of 
 mesoblast covered by hypoblast ; but both open freely in front into the common 
 splanchnic stalk. 
 
 In the next stage it still further alters its position, and forms, Fig. 50, a rather 
 wide vesicle lying immediately below the hind end of the digestive canal, with 
 which it communicates freely by a still broad opening ; its blind end projects 
 freely into the pleuroperitoneal cavity below. It was in this condition when 
 Von Baer first observed it. 
 
 At the time when these changes are taking place, the somatopleure is 
 being folded in to form the walls of the body; and as the folds, one on either 
 side, are carried forward from the extreme end of the tail, they present them- 
 selves, when seen from within or in sections, as two ridges projecting towards 
 the sides of the allantois. Reaching the allantois these ridges fuse with its wall, 
 and in this way reduce the pleuroperitoneal cavity immediately below the allan- 
 tois to quite a narrow space, which is seen in section as a mere chink. Remak 
 apparently mistook these infoldings of the somatopleure, and the consequent 
 projections into the pleuroperitoneal cavity, for the first formation of the 
 allantois, although they have in fact little or no connection with it. 
 
 We may therefore probably consider the , allantois as a 
 portion of the cloaca, which grows forward and becomes an 
 independent spherical vesicle, still however remaining con- 
 nected with the cloaca by a narrow canal which forms its neck 
 or stalk. The opening of the allantois into the cloaca is on 
 the under side of the latter. Both the neck and vesicle of 
 the allantois are lined by hypoblast, while its exterior is com- 
 posed of the mesoblast of the splanchnopleure. From the 
 first the allantois lies in the pleuroperitoneal cavity. In this 
 cavity it grows forwards till it reaches the front limit of the 
 hind-gut, where the splanchnopleure turns back to reach 
 the yolk-sac. It does not during the third day project be- 
 yond this point ; but on the fourth day begins to pass out 
 beyond the body of the chick along the as yet wide space 
 between the splanchnic and somatic stalks of the embryo 
 on its way to that space between the external and internal 
 folds of the amnion, which it will be remembered is directly 
 continuous with the pleuroperitoneal cavity (Fig 8, K). In 
 this space it eventually spreads out over the whole body of 
 the chick. On the first half of the day the vesicle is still 
 very small, and its growth is not very rapid. Its mesoblast 
 wall still remains very thick. In the later half of the day its 
 growth becomes very rapid and it forms a very conspicuous 
 object in a chick of that date (Fig. 46, Al.}. At the same 
 time its blood-vessels become important. To these we shall 
 presently return. 
 
VI.] THE PROTOVEBTEBILE. 151 
 
 11. The proto vertebrae, which by the continued differ- 
 entiation of the axial mesoblast at the tail end of the embryo 
 have increased in number from thirty to forty, undergo during 
 this day changes of great importance. Since these changes 
 are intimately connected with the subsequent development 
 of the vertebral column, it will perhaps be more convenient 
 to describe briefly here the whole series of events through 
 which the protovertebrse become converted into the per- 
 manent structures to which they give rise, though many of 
 the changes do not take place till a much later date than 
 the fourth day. 
 
 The separation of the muscle-plates (Chap. v. 23) left 
 the remainder of each proto vertebra as a somewhat tri- 
 angular mass lying between the neural canal and notochord 
 on the inside, and the muscle-plate and intermediate cell-mass 
 on the outside (Fig. 44). Already on the third day the upper 
 angle of this triangle grows upwards, between its muscle- 
 plate and the neural canal, and meeting its fellow in the 
 middle line above, forms a roof of mesoblast over the 
 neural canal, between it and the superficial epiblast. At 
 about the same time, the inner and lower angle of the 
 triangle grows inwards towards the notochord, and passing 
 both below it (between it and the aorta) and above it (be- 
 tween it and the neural canal), meets a similar growth from 
 its fellow proto vertebra of the other side, and thus com- 
 pletely invests the notochord with a coat of mesoblast, 
 which, as seen in Fig. 47, is at first much thicker on the 
 under than on the upper side. 
 
 While the inner portion of each proto vertebra is thus 
 extending inwards around both notochord and neural canal, 
 the remaining outer portion is undergoing a remarkable 
 change. It becomes divided into an anterior or praeaxial, 
 and a posterior or postaxial segment. The anterior, which is 
 the larger and more transparent of the two, is the rudiment 
 of the spinal ganglion and nerve, while the posterior, which 
 remains more particularly connected with the extensions 
 round the neural canal and notochord, goes to form part of 
 the permanent vertebra. 
 
 In this way, each protovertebra, having given rise to a 
 muscle-plate, is further divided into a ganglionic rudiment, 
 and into a mass which we may speak of as a "primary" 
 
152 THE FOURTH DAY. [CHAP. 
 
 vertebra, consisting as it does of a body or mass investing the 
 notochord, from which springs an arch covering in the neural 
 canal. 
 
 Both body and arch consist at this epoch of but slightly 
 differentiated mesoblast, and the arch springs, to a certain 
 extent, not oily from the posterior segment of the protover- 
 tebra, but also from the anterior or ganglionic segment : 
 though, as seen in Fig. 47, it is far less conspicuous at the 
 level of the latter than of the former. Both neural canal 
 and notochord are thus furnished from neck to tail with a 
 complete investment of mesoblast, still marked, however, by 
 the transparent lines indicating the fore and aft limits of 
 the several protovertebrse. This is sometimes spoken of as 
 the " membranous " vertebral column. 
 
 The ganglionic rudiment, placed anteriorly to its corre- 
 sponding primary vertebra, consists in chief of a large oval 
 swelling, the ganglion of the posterior root (Fig. 47, pr). 
 At a little distance beyond its ganglion, the posterior root is 
 joined by the anterior root (ar) ; and the two form together 
 the common nerve-trunk, which is at first very short. Com- 
 pared with either root or with the nerve-trunk the ganglion, 
 at this epoch and for some time afterwards, seems dispropor- 
 tionably large. At first, neither root is connected with the 
 involuted epiblast of the neural canal. Very speedily, how- 
 ever, they both come to be united with that portion of the 
 neural tube which, as we shall presently state, gives origin 
 to the grey matter of the spinal cord. It is, however, easier 
 to trace the fibres of the anterior root into the cord, than 
 those of the posterior, and they can be followed in it for a 
 greater distance. 
 
 On the fourth day the nerves are composed of cells whose protoplasm is 
 beginning to become converted into fibres. Amongst these fibres, the nuclei 
 of the cells with distinct nucleoli are thickly scattered. On the sixth day and 
 still more on the seventh the fibrillated structure of the nerves is much more 
 distinct and the nuclei are far less numerous. 
 
 The ganglia on the fourth day are composed of numerous nuclei surrounded 
 by protoplasm, between which the fibres of the nerves pass. Covering this mass 
 of nucleated cells is a layer of mesoblast (also derived from the tissue of the 
 protovertebra) which, by the sixth day, forms a kind of sheath around them. 
 
 The cells of the ganglia from the fourth to the sixth day contain round 
 granular nuclei with distinct nucleoli very similar to the nuclei of the 
 ordinary mesoblast-cells. The limits of the protoplasm of the individual cells 
 are as a general rule not easily seen, but with care may be made out. The 
 amount of protoplasm round each nucleus appears however to be very small. 
 
VI.] THE SPINAL GANGLIA. 153 
 
 The fibres of the nerve can easily be traced through the ganglion. In 
 section they appear to have a somewhac wavy course, and by interlacing divide 
 the ganglion into a number of elongated areas in each of which is a row of 
 nuclei. In sections of the sixth day it is not possible to trace a connection 
 between the nerve-fibres and the cells. The nuclei are most numerous at 
 the lower ends of the ganglia. 
 
 On the seventh day, the nuclei have become larger, and where the outline 
 of a cell can be distinctly seen it is generally somewhat angular. In sections 
 it is still on the seventh day not possible to trace any connection between the 
 cells and the nerve-fibres. 
 
 Remak (op. cit.) speaks of the ganglia being composed of non-nucleated 
 spheres, and Lockhart Clarke (Philosopkical Transactions, 1862) also describes 
 the ganglia-cells as "cells or nuclei" which are at first mere rounded masses 
 of protoplasm, and do not acquire a nucleus till a later period. Both of these 
 statements are according to our observations incorrect. 
 
 In the later stages according to Lockhart Clarke's account (loc. cit.) the cells 
 of the ganglia send out processes which anastomose together into a fine 
 network. The ceils also become connected with the nerve fibres, which can 
 sometimes be seen to divide in the ganglion into a fine brush-like bundle of 
 fibrillae. At this time the cells possess a distinct nucleus and nucleolus. 
 These changes he describes aa completed by the ninth day of incubation. 
 
 His believes that the spinal nerves are derived from downward prolonga- 
 tions of the superficial epiblast descending between the pro to vertebrae. This 
 view has not been corroborated by subsequent observers. 
 
 12. The remaining portions of the protovertebrae form- 
 ing the primary vertebrae or membranous vertebral column 
 spoken of in the last paragraph, are converted into the per- 
 manent vertebras ; but their conversion is complicated by a 
 remarkable new or secondary segmentation of the whole 
 vertebral column. 
 
 On the fourth day, the transparent lines marking the 
 fore and aft limits of the proto vertebras are still distinctly 
 visible. On the fifth day, however, they disappear, so that 
 the whole vertebral column becomes fused into a homoge- 
 neous mass whose division into vertebrae is only indicated by 
 the series of ganglia. This fusion, which does not extend to 
 the muscle-plates in which the primary lines of division still 
 remain visible, is quickly followed by a fresh segmentation, 
 the resulting segments being the rudiments of the permanent 
 vertebras. The new segmentation, however, does not follow 
 the lines of the earlier division, but passes between the 
 ganglionic and the vertebral portions, in fact, through the 
 middle, of each protovertebra. In consequence, each spinal 
 ganglion and nerve ceases to form the front portion of the 
 primary vertebra, formed out of same protovertebra as itself, 
 but is attached to the hind part of the permanent vertebra 
 
154 THE FOURTH DAY. [CHAP. 
 
 immediately preceding. Similarly, the rudiment of each 
 vertebral arch covering in the neural tube no longer springs 
 from the hind part of the protovertebra from which it is an 
 outgrowth, but forms the front part of the permanent ver- 
 tebra, to which it henceforward belongs. The ganglia are still, 
 however, the most conspicuous portions of each segment. 
 
 By these changes this remarkable result is brought about, 
 that each permanent vertebra is formed out of portions of 
 two consecutive proto vertebrae. Thus, for instance, the tenth 
 permanent vertebra is formed out of the hind portion of the 
 tenth protovertebra, and the front portion of the eleventh 
 protovertebra, while its arch, now attached to its front 
 part, was attached to the hind part of the tenth protovertebra. 
 
 The new segmentation is associated with or rather is 
 caused by histological changes. At the time when the fusion 
 takes place, the mesoblast, which in the form of processes 
 from the proto vertebral bodies surrounds and invests the 
 notochord, has not only increased in mass but also has 
 become cartilaginous, so that, as Gegenbaur (Untersuchung 
 zur vergleichenden Anatomic der Wirbelsdule bei Amphibien 
 und Reptilien, Leipzig, 18G2) points out, we have for a short 
 period on the fifth day a continuous and unsegmented carti- 
 laginous investment of the notochord. 
 
 This cartilaginous tube does not however long remain uni- 
 form. At a series of points corresponding in number to the 
 original proto vertebrae it becomes connected with a number 
 of cartilaginous arches which appear in the protovertebral 
 investment of the neural canal. These arches, which thus 
 roof in the neural canal and each of which arises opposite to 
 the vertebral portion of each protovertebral body, are the 
 cartilaginous precursors of the osseous vertebral arches. We 
 further find that the portions of the cartilaginous tube from 
 which the arches spring come to differ histologically from the 
 portions between them not connected with arches : they are 
 clearer and their cells are less closely packed. There is 
 however at this period no distinct segmentation of the 
 cartilaginous tube, but merely a want of uniformity in its 
 composition. 
 
 The clearer portions, from which the arches spring, form 
 the bodies of the vertebrce, the segments between them the 
 intervertebral regions of the column. 
 
VI.] SECONDARY SEGMENTATION OF VERTEBRAL COLUMN. 155 
 
 W. Schwarck (Beitrdge zur Entwicldung der Wirbelstiule ~bd den Vogeln. 
 Anatomische Studien, Dr JJasse, in. Heft, 1872) states that both in the 
 intervertebral and the vertebral segments the cartilage is divided into two 
 layers, an inner, central, and an outer peripheral. This division is less marked 
 in the intervertebral than in the vertebral region. 
 
 The inner layer in the vertebral region he speaks of as "the body of the 
 vertebra belonging to the notochord," and the external layer as "the skeleton- 
 forming layer." 
 
 On the fifth day a division takes place of each of the in- 
 tervertebral segments into two unequal parts ; a larger one 
 appertaining to the vertebra in front and a smaller one to. the 
 vertebra behind. To the larger segment the spinal ganglia 
 naturally remain attached, and thus comes about the altera- 
 tion of their place in relation to the vertebrae which we before 
 spoke of. 
 
 This fresh segmentation is not well marked, if indeed it 
 takes place at all in the sacral region. 
 
 Each arch at its first appearance corresponds to about the 
 middle of a vertebral portion of a proto vertebra, but after the 
 secondary segmentation the portion of each vertebra behind 
 its arch grows more quickly than that in front, and thus after 
 a while the arches seem to spring from the front rather than 
 from the middle of the vertebral segments. 
 
 To recapitulate: the original protovertebras lying side by 
 side along the notochord, after giving off the muscle-plates, and 
 dividing lengthways into ganglionic and vertebral portions, 
 grow around, and by fusing together completely invest, 
 with mesoblast of protovertebral origin, both neural canal 
 and notochord. 
 
 This investment, of which by reason of its greater growth 
 the original bodies of the protovertebrse seem to be only an out- 
 lying part, becomes cartilaginous in such a way that while the 
 notochord becomes surrounded with a thick tube of cartilage 
 bearing no signs of segmentation, but having the ganglia 
 lodged on its exterior at intervals, the neural canal is covered 
 in with a series of cartilaginous arches, connected to each 
 other by ordinary mesoblastic tissue only, but passing at their 
 bases directly into the cartilaginous tube around the noto- 
 chord. 
 
 By a histological process of differentiation the cartila- 
 ginous tube is divided into vertebral and intervertebral 
 portions, the vertebral portions corresponding to the arches 
 
156 THE FOURTH DAY* [CHAP. 
 
 over the neural canal. Fresh lines of segmentation then 
 appear in the intervertebral portions, which run in. such a 
 way that each ganglion is now more closely associated with the 
 vertebral portion in front of it than with that behind it, 
 though the latter sprang in part from the same original 
 protovertebra as itself. 
 
 13. Meanwhile from the fourth to the sixth day im- 
 portant changes take place in the notochord itself. 
 
 On its first appearance the notochord was, as we have seen, 
 composed of somewhat radiately arranged but otherwise 
 perfectly typical mesoblast-cells. 
 
 On the third day some of the central cells become vacuo- 
 lated, while the peripheral cells are still normal. The vacuo- 
 lated cells exhibit around the vacuole a peripheral layer of 
 granular protoplasm in which the nucleus lies embedded, 
 whilst the vacuoles themselves are filled with a perfectly 
 clear and transparent material, which in an unaltered con- 
 dition is probably fluid. Towards the end of the day the 
 notochord acquires a delicate structureless sheath which is no 
 doubt a product of its peripheral cells. 
 
 According to His there is a cavity in the centre of the notochord on the 
 third day. We have never observed this, arid . it is denied by Miiller 
 (Ueber den Bau der Chorda Dorsalis. Jenaische Zeitschrift. Bd. vi. 1871). 
 
 On the fourth day all the cells become vacuolated with 
 the exception of a single layer of flattened cells at the peri- 
 phery ; and the vacuoles themselves become larger. At the 
 point where the nucleus lies there is generally rather more 
 protoplasm than round the remainder of the circumference 
 of the cells. 
 
 On the sixth day all the cells are vacuolated. In each cell 
 the vacuoles have so much increased at the expense of the 
 protoplasm that only a very thin layer of the latter is left at 
 the circumference of the cell, at one part of which, where 
 there is generally more protoplasm than elsewhere, the 
 starved remains of a nucleus may generally be detected. 
 
 Miiller (loc. cit.) considers that the cells have a membrane. This however 
 is probably merely a hardened external layer of the protoplasm ; and is stained 
 by reagents. 
 
 Dursy (Zur EntwicldungsgeschicJite des Kopfes des Menscken und der hoheren 
 Wirbeltkiere) believes that what we have spoken of as vacuoles in the cells are 
 really intercellular spaces. So that according to his view the notochord is 
 composed of stellate cells with large round intercellular spaces filled with 
 transparent intercellular matter. Superficially viewed a section of the noto- 
 
VI.] THE NOTOCHORD. 157 
 
 chord of the sixth day might be supposed to have such a structure, but the study 
 of its development and a careful examination of its structure proves that this is 
 not a correct account. 
 
 According to the measurements of Miiller (loc. cit.} the diameter of the 
 notochord on the third day is o'op mm. and that of the central cells o'Ois 
 0*018. On the fourth day the notochord is 0*16 mm. in diameter and its com- 
 ponent cells are also larger. On the sixth day its diameter is at the maximum, 
 and reaches 0*2 inm. The central cells measure 0*02 mm. 
 
 From these measurements it will be seen that the vacuolation of the cells 
 of the notochord is accompanied by a rapid growth both in the size of the cells 
 and in the diameter of the notochord itself. 
 
 14. The notochord is on the sixth day at the maximum of 
 its development, the change which it henceforward under- 
 goes being of a retrograde character. 
 
 From the seventh day onward, it is at various points 
 encroached upon by its investment. Constrictions are thus 
 produced which first make their appearance in the interverte- 
 bral portions of the sacral region. In the cervical region, 
 according to Gegenbaur, the intervertebral portions are not 
 constricted till the ninth day, though as early as the seventh 
 day constrictions are visible in the vertebral portions of 
 the lower cervical vertebrae. By the ninth and tenth days, 
 however, all the intervertebral portions have become distinctly 
 constricted, and at the same time in each vertebral portion 
 there have also appeared two constrictions giving rise to a 
 central and to two terminal enlargements. In the space 
 therefore corresponding to each vertebra and its appropriate 
 intervertebral portion, there are in all four constrictions and 
 three enlargements. 
 
 On the twelfth day the ossification of the bodies com- 
 mences. At that time, according to Schwarck (loc. cit.}, the 
 cartilaginous bodies of the vertebras are composed of an inner 
 layer in which the cells form lines radiating from the noto- 
 chord, and an outer layer somewhat sharply separated from 
 the inner one. In the inner layer, immediately around the 
 notochord, ossification first commences. 
 
 Gegenbaur (loc. cit. p. 67) considers that this layer in which ossification 
 commences corresponds to the primordial body of the vertebrae in amphibians. 
 Schwarck is doubtful whether it corresponds to his inner layer of cartilage in 
 the first stage. 
 
 In rare cases ossification first commences as a deposit on the exterior of the 
 vertebrae. 
 
 The first vertebra to ossify is the second or third cervical, 
 and the ossification gradually extends backwards. It does 
 
158 THE FOUBTH DAY. [CHAP. 
 
 not commence in the arches till somewhat later than in the 
 bodies. For each arch there are two centres of ossification, 
 one on each side. 
 
 We may remind the reader that in the adult bird we find between each of the 
 vertebrae of a neck and back a cartilaginous disc the meniscus which is 
 pierced in the centre. These discs are thick at the circumference but thin 
 off to a fine edge round the central hole. Owing to the shape of these 
 discs there are left between each pair of vertebrae two cavities, which only 
 communicate through the central aperture of the meniscus. Through this 
 central aperture there passes a band connecting the two vertebrae which is called 
 the 'ligamentum suspensorium..' 
 
 In the tail the menisci are replaced by bodies known as the 'annuli fibrosi,' 
 which precisely resemble the similarly named bodies in mammals. They differ 
 from the menisci in being attached over their whole surface to the ends of the 
 vertebral bodies, so that the cavities between the menisci and the vertebrae 
 cease to exist. They are pierced however by a body corresponding with the 
 ligamentum suspensorium and knoun as the 'nucleus pulposus.' 
 
 In the iritervertebral regions the chorda, soon after the commencement of 
 ossification, entirely disappears. The cartilage around it however becomes 
 converted (in the region of the neck) into the ligarnentum suspensorium, 
 which unites the two vertebrae between which it is placed. 
 
 In the tail the cartilage becomes the nucleus pulposus, which corresponds 
 exactly to the * ligamentum suspensorium' of the neck arid back. 
 
 Shortly after the formation of the ligamentum suspensorium the remaining 
 cartilage of the intervertebral segments is converted in the neck and back 
 into the meniscus between each two vertebrap, and in the tail into the annulus 
 fibrosus. Both are absent in the sacrum. These points together with the 
 anatomy of these parts in the adult were first made out by Jager ( Wirbelkorper- 
 gelenk der Vogel. Sitz. der k. Akad. Wien, vol. xxxin. 1859). 
 
 In the bodies of the vertebrae the notochord does not entirely disappear as 
 in the intervertebral regions, but, accord ing to Gegenbaur, undergoes ultimately 
 a direct conversion into cartilage. The contour of the sheath becomes 
 indistinct ; the cells by the accumulation of matrix round them take on the 
 form of cartilage-cells, so that at the time of the exclusion of the bird from the 
 egg the limits between the altered notochord and the cartilage of protovertebral 
 origin can only with difficulty be made out. 
 
 15. While the chief mass of a proto vertebra, having 
 given rise to a muscle-plate and a ganglion, is converted into 
 the body and arch of a permanent vertebra with its several 
 appurtenances, a small portion of the exterior grows down- 
 wards as the rudiment for the formation of a rib. These 
 costal growths are of course confined to the dorsal region. 
 They are seen on the sixth day as cartilaginous rods, whose 
 cells are arranged in horizontal rows. By this time they 
 are quite separate from the bodies of the vertebras, with 
 whose arches they are in transverse section seen to alternate. 
 Thus in one section the vertebral arch will be distinctly seen 
 but no trace of the rib ; while in the next the rib will be 
 visible but the arch will be absent. 
 
VI.] THE MUSCLE-PLATES. 159 
 
 16* We shall conclude our account of the proto vertebra 
 by describing the changes which take place in the muscle- 
 plates. 
 
 In the chick these are somewhat complicated, and have 
 not been fully worked out. 
 
 On the third day the muscle-plates end opposite the 
 point where the mesoblast becomes split into somatopleure 
 and splanchnopleure. OD the fourth day however (Fig. 
 47 mp.) they extend to a certain distance into the side walls 
 of the body beyond the point of the division into somatopleure 
 and splanchnopleure. 
 
 Into what muscles of the trunk they become converted has 
 been somewhat disputed. There is no doubt that it is only epi- 
 skeletal muscles, to use Professor Huxley's term (Vertebrates, 
 p. 46), that are derived from them, but some embryologists 
 have stated that they only form the muscles of the back. 
 We have, however, little doubt that all the episkeletal 
 muscles are their products ; a view also adopted by Professors 
 Huxley and Kolliker. 
 
 According to Kolliker the milscle-plates give rise to (i) the deep dorsal 
 muscles, such as the semispinalis multifidus &c., and (2) the visceral muscles 
 as represented by abdominal muscles, the muscles of the breast, the superficial 
 muscles of the neck, and the muscles of the jaws and face. 
 
 The front dorso-lateral (hyposkeletal) muscles, according to Kolliker, are 
 derived from a front (ventral) muscle-plate, which is formed from the most 
 ventral portion of the protovertebrse, but is very limited in extent in the 
 fowl. These muscles include the longus colli, the recti antici, and quadratus. 
 
 This view differs from that of Huxley, chiefly in considering only the 
 ventral dorsal muscles as hyposkeletal, and not also the inner visceral muscles. 
 Huxley believes that all the episkeletal muscles are derived from the muscle- 
 plates, but does not give an opinion as to the cells of the embryo from 
 which the hyposkeletal muscles take their origin. 
 
 His takes an entirely different view ; he believes that the muscles of the back 
 only are derived from the muscle-plates, but that the muscles of the sides and 
 ventral walls of the body are formed from the mesoblast of the somatopleure. 
 
 There can be little doubt that the intrinsic muscles of the limbs are not out- 
 growths from the muscle-plates, but are formed independently in the meso- 
 blastic tissues of which the limbs are composed. 
 
 The origin of the extrinsic limb-muscles is not so certainly known. 
 
 The cutaneous muscles are obviously derived from the original mesoblast of 
 the somatopleure. 
 
 It seems very probable (though the subject has not yet been worked out) 
 that the hyposkeletal voluntary muscles underlying the vertebral column are 
 derived from the intermediate cell-mass, which originally lies externally to the 
 protovertebrae, but into which, as we have before said, the cleavage of the 
 mesoblast does not extend. 
 
 In the first instance, as is clear from their mode of origin, the muscle-plates 
 correspond in number with the protovertebrse, and this condition is permanent 
 
160 THE FOURTH DAY. [CHAP. 
 
 in the lower vertebrates, such as fishes, where we find that the lateral muscle is 
 divided by septa into a series of segments corresponding in number with the 
 vertebrae. 
 
 17. Of all the events of the fourth day, none perhaps are 
 more important than those by which the rudiments of the 
 complex urinary and generative systems are added to the 
 simple Wolffian duct and body, which up to that time are 
 the sole representatives of both systems. 
 
 We saw that the duct arose on the second day as a solid 
 ridge which subsequently became a tube, lying immediately 
 underneath the epiblast above the intermediate cell-mass, 
 close against the upper and outer angles of the proto- 
 vertebrse, and reaching from about opposite to the fifth 
 protovertebra away to the hinder end of the embryo. 
 
 The exact manner in which it first appears is as yet a 
 matter of dispute, and in our account of the second day, we 
 gave the views of the majority of embryologists who have 
 written on the subject. But it may be considered as quite 
 certain that the Wolffian duct is formed out of mesoblast- 
 cells. It is most probable that the ridge is primarily formed 
 by simple aggregation of cells, and that it is converted into 
 a tube by its central cells taking on a radiating arrange- 
 ment round a central hole, which is at first small but rapidly 
 increases in size. In whatever way it be really formed, we 
 find before the end of the second day, in the place of the 
 previous ridge, a duct with a distinct though small lumen. 
 Waldeyer and some other observers have incorrectly stated 
 that the lumen is not formed till somewhat later. 
 
 At first the duct occupies a position immediately under- 
 neath the superficial epiblast, but very soon after its forma- 
 tion the growth of the protovertebrae and the changes which 
 take place in the intermediate cell-mass, together with the 
 general folding in of the body, cause it to appear to change 
 its place and travel downwards (Chap. V. 26). While this 
 shifting is going on, the cells lining the upper end of the 
 pleuroperitoneal cavity (the kind of bay which, as seen in 
 sections, is formed by the divergence of the somatopleure and 
 splanchnopleure) become columnar, and constitute a distinct 
 epithelium. This epithelium, which is clearly shewn in Fig. 
 41, g. e, and is also indicated in Fig. 44, is often called i\\Q ger- 
 minal epithelium, because some of its cells subsequently take 
 
VI.] THE WOLFFIAN BODY. 161 
 
 part in the formation of the ovary. Soon after its appearance, 
 the intermediate cell-mass increases in size and begins to 
 grow outwards into the pleuroperitoneal cavity, as a rounded 
 projection which lies with its upper surface towards the 
 somatopleure, and its lower surface towards the splanchno- 
 pleure, but is in either case separated from these layers 
 by a narrow chink. The Wolffian duct (Fig. 44, Wd, 47, 
 Wd) travels down, and finally before the end of the third 
 day is found in the upper part of this projection, near that 
 face of it which is turned towards the somatopleure. 
 
 At, or before, the fourth day, when the duct occupies 
 this new position, the Wolffian body begins to be formed in 
 the midst of the intermediate cell-mass. 
 
 The structure of the fully developed Wolffian body is 
 fundamentally similar to that of the permanent kidneys, and 
 consists essentially of convoluted tubules, commencing in 
 Malpighian bodies with vascular glomeruli, and opening 
 into the duct. It is formed as follows. 
 
 From the anterior portion of each duct and on its inner 
 side, diverticula are given out at right angles. These 
 gradually lengthen, and becoming twisted form the tubules, 
 while the glomeruli of the Malpighian corpuscles seem to be 
 derived from cells of the intermediate mass, which also gives 
 rise to the vascular networks round the tubules. 
 
 The tubules, which from their contorted course are in 
 sections (Figs. 47, 51) seen cut at various angles, possess 
 an epithelium which is thicker than that of the Wolffian 
 duct. From this difference it is generally easy to distin- 
 guish the sections of the tubules from those of the duct. 
 The glomeruli of the Malpighian bodies are in sections of 
 hardened embryos usually filled with blood-corpuscles. 
 
 In the above statements we have followed Waldeyer (EierstocJc und Hi), but 
 it ought to be mentioned that the majority of earlier observers have believed 
 that the tubules arise independently in the mesoblast, and only at a later period 
 become connected with the duct. The sections which Waldeyer has drawn 
 seem however strongly to support the view which he has brought forward ; 
 our own sections also confirm it, and we have noticed that even before the 
 formation of the tubules, the Wolffian duct exhibits great variations in diameter, 
 being in some cases crescent-shaped in section, in others round ; this seems 
 clearly to indicate the giving off of diverticula. Waldeyer's observations have 
 moreover been since confirmed by other observers. 
 
 The Wolffian body, as distinct from the duct, reaches 
 from about the level of the fifth protovertebra to beyond the 
 
 E. 11 
 
162 THE FOURTH DAY. [CHAP. 
 
 hind limbs ; but the duct itself is carried on still further 
 back. 
 
 Towards the hind end of the embryo, the projection of 
 the intermediate cell-mass spoken of above becomes smaller 
 and smaller, and the Wolffian duct is thus brought nearer to 
 the splanchnopleure, and in the region of the hind-gut comes 
 to lie close to the walls of the alimentary canal. On the fourth 
 day, the two ducts meet and open into two horns, into which 
 the side- walls of the recently formed cloaca are at that time 
 produced, one on either side. 
 
 As we shall afterwards see, the duct of the permanent 
 kidneys and Miiller's duct also fall into these two horns of 
 the cloaca. 
 
 The Wolffian bodies thus constituted perform the offices 
 of kidneys for the greater part of embryonic life. In 
 the chick they disappear before birth ; but in most of the 
 Ichthyopsida they remain for life as the permanent kidneys. 
 
 18. Near the end of the fourth day, on the outer surface 
 of the projection formed by the Wolffian body a furrow is 
 formed immediately below the Wolffian duct by an involution 
 of the germinal epithelium. This furrow, which is shewn at 
 M.d in Fig. 47, deepens, and its walls arch over and unite. 
 In this way a tube is formed, which separates from the 
 germinal epithelium in the same way that the neural tube 
 separated from the external epiblast. It is known as the 
 Duct of Mutter ; of its function we shall speak later on. 
 
 This account of the origin of Muller' s duct is due to Waldeyer (loc. tit.}, 
 whose observations have been confirmed by subsequent inquirers. An exami- 
 nation of our own sections leads us to the same conclusions. 
 
 Dr Sernoff (Centralblatt fur Med. Wiss. 27 Jun. 1874) agrees with 
 Bornhaupt (Untersuchung uber die Entwickelung des Urino-gemtalsy stems beim 
 Hilhnchen) in considering that the duct of Miiller is formed by a simple invo- 
 lution from the pleuroperitoneal cavity which grows backwards in the meso- 
 blast between the Wolffian duct and the germinal epithelium ; and thinks that 
 Waldeyer is in error in supposing the involution to be in the form of an elon- 
 gated furrow. This divergence of opinion is not of great importance compared 
 with the point on which both observers are in agreement, viz. that the duct of 
 Muller is formed by an involution of the germinal epithelium from the pleuro- 
 peritoneal cavity. 
 
 The formation of the duct of Muller takes place from 
 before, backwards; but near the hind end of the embryo, 
 where the germinal epithelium is deficient, the groove to 
 form the duct becomes an involution which, at first solid 
 
VI.] THE PERMANENT KIDNEYS. 163 
 
 but subsequently hollow, bores its way through the meso- 
 blast, and finally appears to unite on the seventh day 
 with the Wolffian duct close to the entrance of the latter 
 into the cloaca. Later on, this state of things becomes 
 altered; the duct of Miiller opens directly into the cloaca 
 without first uniting with the Wolffian duct. Its opening 
 then lies above that of the Wolffian duct, between it and the 
 opening into the cloaca of the true urinary canal, of which 
 we shall speak directly. 
 
 The anterior extremity of the duct of Miiller which lies 
 about on a level with the fifth protovertebra is never closed 
 in. Here the original furrow remains open, and forms a 
 funnel-shaped opening into the tube from the pleuro- 
 peritoneal cavity. In sections of the sixth day the duct 
 of Miiller is to be seen lying between the duct of the 
 Wolffian body and the pleuroperitoneal cavity. Its diameter 
 is generally smaller than that of the Wolffian duct. 
 
 19. Between the 80th and 100th hour of incubation, the 
 permanent kidneys begin to make their appearance, and as 
 is the case with the Wolffian bodies, the first portion of them 
 to appear is their duct. Near its posterior extremity the 
 Wolffian duct becomes expanded, and from the expanded 
 portion a diverticulum is constricted off which in a trans- 
 verse section lies above the original duct, and the blind end 
 of which points forwards, that is, towards the head of the 
 chick. This is the duct of the permanent kidney or ureter. 
 At first the ureter and the Wolffian duct open by a common 
 trunk into the cloaca, but this state of things lasts for a 
 short time only, and by the sixth day the two ducts have 
 independent openings. 
 
 The earlier state of things was overlooked by Remak, who thus came to give 
 an incorrect account of the origin of the duct of the kidneys. 
 
 Knpffer (Untersuchung uber die Entwickelung des Ham- und GeschlecTits- 
 systems, Archiv fur Microscop. Anat. Vol. n. 1866) was the first to give a 
 correct account of the development of the duct of the permanent kidneys 
 in the chick. His observations have since been confirmed by a number of other 
 observers, including Waldeyer. 
 
 In sections of a somewhat later period the duct of the 
 kidneys can be seen to lie above (dorsal to) the Wolffian 
 duct, and farther from it than the duct of Miiller. 
 
 The formation of the kidneys themselves is very similar 
 to the formation of the Wolffian bodies, 
 
 11 9 
 
164 THE FOUKTH DAY. [CHAP. 
 
 From the upper end of the ureter diverticula are given 
 off at right angles into the intermediate cell-mass. These 
 lengthening and becoming twisted, form the tubuli urini- 
 feri, while the mesoblast around their extremities becomes 
 directly converted into the Malpighian bodies and the 
 capillary network of the kidneys. Corresponding to the 
 relative position of their ducts, the kidney lies above the 
 Wolffian body. At its first appearance it forms an oval 
 body, lying in the upper part of intermediate cell-mass 
 between the Wolffian body and the vertebral column, and 
 is placed rather nearer the median line than the Wolffian 
 body. 
 
 The formation of the kidneys takes place before the end 
 of the seventh day, but they do not become of functional 
 importance till considerably later. 
 
 From their mode of development it clearly follows that 
 the permanent kidneys are merely parts of the same system 
 as the Wolffian bodies, and that their separation from these 
 is an occurrence of a purely secondary importance. 
 
 20. Before describing the subsequent fate of the Wolffian 
 and Mullerian ducts, it will be necessary to give an account 
 of the formation of the true sexual glands, the ovaries and 
 testes. 
 
 At the first appearance of the projection from the inter- 
 mediate mass, which we may now call the genital ridge, 
 a columnar character is not only visible in the layer of cells 
 covering the nascent ridge itself along its whole length, but 
 may also be traced for some little distance outwards on either 
 side of the ridge in the cells lining the most median portions 
 of both somatopleure and splanchnopleure. Passing out- 
 wards along these layers, the columnar cells gradually give 
 place to a flat tesselated epithelium. As the ridge con- 
 tinues to increase and project, the columnar character be- 
 comes more and more restricted to cells covering the ridge 
 itself, in which at the same time it becomes more distinct. 
 On the outer side of the ridge, that is on the side which 
 looks towards the somatopleure, the epithelium undergoes, as 
 we have seen, an involution to form the duct of Muller, and 
 for some little time retains in the immediate neighbourhood 
 of that duct its columnar character (Fig. 51, a'), though 
 eventually losing it. 
 
THE .GERMINAL EPITHELIUM. 
 
 165 
 
 The median portion of the ridge is occupied by the 
 projection of the Wolffian body, and here the epithelium 
 rapidly becomes flattened. 
 
 On the inside of the ridge, however, that is on the side 
 looking towards the splanchnopleure, the epithelium not only 
 retains its columnar character, but grows several cells deep 
 (Fig. 51, a), while at the same time the mesoblast (E) under- 
 lying it becomes thickened. In this way, owing partly to 
 the increasing thickness of the epithelium, and partly to 
 the accumulation of mesoblast beneath it, a slight eminence 
 is formed, which when viewed from above, after opening the 
 
 SECTION OF THE INTERMEDIATE CELL-MASS ON THE FOUBTH DAY. (Prom 
 Waldeyer.) Magnified 160 times. 
 
 m. mesentery. L. somatopleure. a', portion of the germinal epithelium from 
 which the involution to form the duct of Miiller (2) takes place, a. thickened 
 portion of the germinal epithelium in which the primitive ova C and o are 
 lying. E. modified mesoblast which will form the stroma of the ovary. 
 WK. Wolffian body. y. WolfEan duct. 
 
166 THE FOURTH DAY. [CHAP. 
 
 abdominal cavity, appears in direct light as a fusiform white 
 patch or streak, in its early stages extending along the whole 
 length of the Wolffian body and genital ridge, but sub- 
 sequently restricted to its anterior portion. Its appearance 
 under these circumstances has been well described by Von Baer. 
 This 'sexual eminence' is present in the early stages of 
 both sexes. In both the epithelium consists of several layers 
 of short cylindrical cells, a few of which are conspicuous on 
 account of their size and their possessing a highly refractive 
 oval nucleus of considerable bulk ; in both, the underlying 
 thickened mesoblast consists as indeed at this epoch it does 
 generally in all parts of the body of spindle-shaped cells. 
 
 The larger conspicuous cells of the epithelium, which 
 appear to have quite a common origin with their fellow cells, 
 and to rise from them by direct differentiation, and which 
 are seen at the first in male as well as female embryos, are 
 the primordial ova (Fig. 51, o). Thus in quite early stages 
 it is impossible to detect the one sex from the other. At 
 about the 80th to the 100th hour, however, a distinction 
 becomes apparent. 
 
 In the males, the epithelium with its underlying meso- 
 blast ceases to develope ; the primordial ova neither increase 
 nor multiply. On the contrary, they disappear, and the 
 whole sexual eminence fades away. 
 
 In females, on the other hand, the primordial ova enlarge 
 and become more numerous, the whole epithelium growing 
 thicker and more prominent. The spindle-shaped cells of 
 the underlying mesoblast also increase rapidly, and thus 
 form the stroma of the ovary. The growth of this stroma 
 bears subsequently such a relation to that of the epithelium, 
 that the primordial ova appear to sink into the stroma, and 
 each ovum, as it descends, to carry with it a number of the 
 ordinary epithelium-cells, which arrange themselves round it 
 in a distinct layer. In this way each ovum becomes invested 
 by a capsule of vascular connective tissue, lined internally by 
 a layer of epithelium ; the whole constituting a Graffian 
 follicle. The large nucleus of the primordial ovum becomes 
 the germinal vesicle, while the ovum itself remains as the 
 true ovum ; this subsequently becomes enlarged by the ad- 
 dition of a quantity of yolk derived from the epithelial lining 
 of the follicle. 
 
VI.] THE TESTES. 167 
 
 Pfliiger (Die Eierstocke der Sdugethiere u. des Menschen, Leipzig, 1863) de- 
 scribed the ova as arising, in mammals, out of the epithelium of tubular glands, 
 a chain of several ova being frequently found in one tube and the tube be- 
 coming subsequently divided by constrictions into as many follicles. According 
 to Waldeyer however, whose account we have followed above, the primordial 
 ova make their appearance as individual specialized epithelium- cells, without the 
 preformation of any tubular glands, the capsule or Graffian follicle being a later 
 product. Waldeyer's views have been on the whole generally accepted (Leo- 
 pold, Untersuch. uber das Epithel. des Ovariums. Inaug. Diss. Leipzig, 1870, 
 Bomiti, Max Schnitzels Archiv, 1873, Bd. x.), though opposed by Kapff (Rei- 
 chert and Du ois Reymond's Archiv, 1872), and more recently by Sernoff 
 (loc. cit.). 
 
 The first traces of the testes are found in the dorsal and 
 inner side of the intermediate cell-mass, and appear about the 
 sixth day. From the first they differ from the rudimentary 
 ovaries, by coming into somewhat close connection with 
 the Wolffian bodies ; but occupy about the same limits from 
 before backwards. The mesoblast in the position we have 
 mentioned begins to become somewhat modified, and by the 
 eighth day is divided by septa of connective tissue into a 
 number of groups of cells ; which are the commencing tubuli 
 seminiferi. By the sixteenth day the cells of the tubuli have 
 become larger and acquired a distinctly epithelial character. 
 
 Waldeyer is of opinion that the tubules of the Wolffian body penetrate into 
 the tissue from which the testes are formed, and becoming much finer than the 
 remainder of the tubules constitute the Hubuli seminiferi.' Apart from its 
 inherent difficulties, this view has not been corroborated by any subsequent 
 observer. 
 
 It is distinctly denied by Sernoff (loc. cit.}, who further states that the testes 
 are entirely formed out of the mesoblast of the intermediate cell-mass, and that 
 their rudiments have no connection either with the germinal epithelium or with 
 the tubules of the Wolffian body. 
 
 We have now described the origin of all the parts which 
 form the urinary and sexual systems, both of the embryo and 
 adult. It merely remains to speak briefly of the changes, 
 which on the attainment of the adult condition take place in 
 the parts described. 
 
 The Wolffian body, according to Waldeyer, may be said 
 to consist of a sexual and urinary part, which can, he states, 
 be easily distinguished in the just-hatched chick. The sexual 
 part becomes in the cock the after-testes or coni vasculosi, 
 and consists of tubules which lose themselves on the one 
 hand in the seminiferous tubules, and on the other hand, in 
 birds, probably form the whole of what can be called the 
 epididymis. In the hea it forms part of the parovarium of 
 
168 THE FOURTH DAY. [CHAP. 
 
 His, and is composed of well-developed tubes without pig- 
 ment. The urinary part forms in both sexes a small rudi- 
 ment, consisting of blindly ending tubes with yellow pigment, 
 but is most conspicuous in the hen. 
 
 The Wolffian duct remains as the vas deferens in the 
 male. In the female it becomes atrophied and nearly dis- 
 appears. 
 
 The duct of Muller on the right side (that on the left side 
 with the corresponding ovary generally disappearing) remains 
 in the female as the oviduct. In the male it is almost 
 entirely obliterated on both sides. 
 
 21. We may return to the changes which are taking 
 place in the circulation. 
 
 On the fourth day, the point at which the dorsal aorta 
 divides into the two branches which we may now call the iliac 
 arteries is carried much further back towards the tail. 
 
 A short way beyond the point of bifurcation, each iliac 
 gives off a branch to the newly formed allantois. It is not, 
 however, till the second half of the fourth day, when the 
 allantois grows rapidly, that these allantoic, or as we may now 
 call them umbilical, arteries acquire any importance, if indeed 
 they are present before. With the increase of the allantois 
 they speedily acquire such a size, that the iliac trunks from 
 which they were given off seem to be mere branches of them- 
 selves. 
 
 The omphalo-mesaraic arteries are before the end of the 
 day given off from the undivided aortic trunk as a single but 
 quickly bifurcating vessel, the left of the two branches 
 into which it divides being much larger than the right. 
 
 During the third day, we saw that the arterial arch 
 running in the first visceral fold became obliterated, the 
 obliteration being accompanied by the appearance of a new 
 (fourth) arch running in the fourth visceral fold on either 
 side. 
 
 During the fourth day the second pair of arterial arches 
 also becomes nearly (if not entirely) obliterated; but a new 
 pair of arches is developed in the last (fifth) visceral fold, 
 behind the last visceral cleft; so that there are still three 
 pairs of arterial arches, which however now run in the 
 third, fourth and fifth visceral folds. The last of these is as 
 yet small, and together with the slight remains of the second 
 
VI.] THE ARTERIAL ARCHES. 169 
 
 pair of arches we may consider that there are in all four 
 pairs of arches. When the first and second arches are 
 obliterated, it is only the central portion of each arch on 
 either side which absolutely disappears. The ventral portion 
 connected with the bulbus arteriosus, and the dorsal portion 
 which joins the dorsal aorta, both remain, and are both 
 carried straight forward towards the head. The ventral 
 portions of both first and second arches unite on each side to 
 form a branch, the external carotid (Fig. 52, E. CA), which 
 runs straight up from the bulbus arteriosus to the head. 
 
 FIG. 52. 
 
 JE.C+. 
 
 T.CA 
 
 STATE OF ARTERIAL CIRCULATION ON THE FIFTH OR SIXTH DAT. 
 
 E. CA. external carotid. /. CA. internal carotid. AO. dorsal aorta, wf. A. 
 arteries to the Wolffian bodies. Ver. A. arteries given off between each of 
 the vertebrae. Of. A, omphalo-mesaraic artery. UA. umbilical artery. 
 I A. iliac artery. 
 
 In the same way the dorsal portions form a branch, the 
 internal carotid, which takes its origin from the dorsal or far 
 end of the third arch. 
 
 22. In the venous system important changes also occur. 
 
 As the liver in the course of its formation wraps round 
 the common trunk of the omphalo-mesaraic veins, or meatus 
 venosus, it may be said to divide that vessel into two parts : 
 
170 
 
 THE FOURTH DAY. 
 
 [CHAP. 
 
 into a part nearer the heart which is called the sinus venosus 
 (Fig. 53, S.V.\ and into a part surrounded by the liver 
 which is called the ductus venosus. Beyond, i.e. behind the 
 liver, the ductus venosus is directly continuous with the 
 omphalo-mesaraic veins, or as we may now say, vein, for the 
 right trunk has become so small as to appear a mere branch 
 of the left. (Fig. 53, Of.) 
 
 We saw that on the third day the ductus venosus, while 
 running through the liver, exhibited numerous 
 
 bulgmgs 
 
 FIG. 53. 
 
 DlAGBAM OF THE VENOUS ClKCULATION AT THE COMMENCEMENT OF THE FlFTH 
 
 DAY. 
 
 H. heart. 2). O. ductus Cuvieri. Into the ductus Cuvieri of each side fall /. the 
 jugular vein, or superior cardinal vein, Su. V. the superior vertebral vein, W. 
 the vein from the wing and C. the inferior cardinal vein. S. V. sinus 
 venosus. Of. omphalo-mesaraic vein. U. umbilical vein, which at this 
 stage gives off branches to the body- walls. 
 
VI.] THE VEINS OF THE LIVER. 171 
 
 indicative of branches about to be formed. These are on the 
 fourth day actually formed, and become connected with the 
 capillary network simultaneously developed in the hepatic 
 substance in such a way that those branches which come off 
 from the ductus venosus soon after its entrance between the 
 liver-lobes, carry blood into the substance of the liver, while 
 those which join the ductus venosus shortly before it leaves the 
 liver, carry blood away from the hepatic substance into the 
 ductus. The former are called vence advehentes, the latter 
 vence revehentes. As a result of this arrangement, there is a 
 choice of paths for the blood in passing from the omphalo- 
 mesaraic vein to the sinus venosus ; it may pass through the 
 capillary network of the liver, going in by the venae adve- 
 hentes, and coming back again by the vena3 revehentes, or it 
 may go straight through the ductus venosus without passing 
 at all into the substance of the liver. 
 
 As the alimentary canal by its continued closing in 
 becomes on the fourth day more and more distinct from the 
 yolk-sac, it gradually acquires veins of its own, the mesenterio 
 veins, which first appear as small branches of the omphalo- 
 mesaraic vein, though eventually, owing to the change in the 
 relative size and importance of the yolk-sac and intestine, the 
 latter seems to be a branch of one of the former. 
 
 Corresponding to the increase in the size of the head, the 
 superior cardinal veins (Fig. 53, */.) become larger and more 
 important and are joined by the superior vertebral (Su.V.) 
 and wing veins (W). As before, they form the ductus 
 Cuvieri (D.C.) by joining with the cardinal veins (C). 
 
 The latter are now largely developed ; they seem to take 
 origin from the Wolffian bodies, and their size and importance 
 is in direct proportion to the prominence of these bodies. 
 They might be called the veins of the Wolffian bodies. 
 
 As the kidneys begin to be formed, a new single median 
 vein makes its appearance, running from them forwards, 
 beneath the vertebral column, to fall into the sinus venosus 
 (Fig. 53, V.C.I.). This is the vena cava inferior. 
 
 As the lungs are being formed, the pulmonary veins also 
 make their appearance and become connected with the left 
 side of the auricular division of the heart. 
 
 The blood carried to the allantois by the umbilical 
 arteries is brought back by two veins which very soon after 
 
172 THE FOURTH DAY. [CHAP. 
 
 their appearance unite close to the allantois into a single 
 trunk, the umbilical vein, which, running along the splanch- 
 nopleure, falls into the omphalo-mesaraic vein (Fig. 53, U). 
 
 23. Meanwhile the heart is undergoing considerable 
 changes. Though the whole organ still exhibits a marked 
 curvature to the right, the ventricular portion becomes 
 directed more distinctly downwards, forming a blunted cone 
 whose apex will eventually become the apex of the adult 
 heart. 
 
 The concave (or dorsal) walls of the ventricles become 
 much thicker, as did the convex or ventral walls on the third 
 day. 
 
 Well-marked constrictions now separate the ventricles 
 from the bulbus arteriosus on the one hand, and from 
 the auricles on the other. The latter constriction is very 
 distinct, and receives the name of canalis auricularis 
 (Fig. 54, CL4.) ; the former, sometimes called the /return 
 Hcdleri, is far less conspicuous. 
 
 The most important event is perhaps the formation of 
 the ventricular septum. This, which commenced on the 
 third day as a crescentic ridge or fold springing from the 
 convex or ventral side of the rounded ventricular portion of 
 the heart, now grows rapidly across the ventricular cavity 
 towards the concave or dorsal side. It thus forms an in- 
 complete longitudinal partition extending from the canalis 
 auricularis to the commencement of the bulbus arteriosus, 
 and dividing the twisted ventricular tube into two somewhat 
 
 FIG. 54. 
 
 C.A 
 
 HEART OF A CHICK ON THE FOURTH DAY OP INCUBATION VIEWED FROM 
 THE VENTRAL SURFACE. 
 
 1. a. left auricular appendage. C. A. canalis auricularis. V. ventricle, b. bulbus 
 - - - .... .... ; arteriosus. 
 
VI.] THE VENTRICULAR SEPTUM. 173 
 
 curved canals, one more to the left and above, the other to the 
 right and below. These communicate freely with each other, 
 above the free edge of the partition, along its whole length. 
 
 Externally the ventricular portion as yet shews no division 
 into two parts. 
 
 The bulbus arteriosus (Fig. 54, 6) has increased in size, 
 and is now very conspicuous. 
 
 The venous end of the heart is placed still more dorsal, 
 and to the left of the arterial end; its walls are beginning to 
 become thicker. 
 
 The auricles are nearly if not quite as far forward as the 
 ventricles; and the auricular appendages (Fig. 54, l.a.\ which 
 were visible even on the third day, are exceedingly prominent, 
 giving a strongly marked external appearance of a division of 
 the auricular portion of the heart into two chambers; but 
 Von Baer was unable to detect at this date any internal 
 auricular septum. 
 
 24. The chief events of the fourth day are : 
 
 (1) The increase of the cranial and body flexure. 
 
 (2) The increase in the tail-fold. 
 
 (3) The formation of the limbs as local thickenings of 
 the Wolffian ridge. 
 
 (4) The formation of the olfactory grooves. 
 
 (5) The absorption of the partition between the mouth 
 and the throat. 
 
 (6) The formation of the allantois as a diverticulum 
 of the alimentary canal. 
 
 (7) The formation of the spinal ganglia. 
 
 (8) The vacuolation of the cells of the notochord. 
 
 (9) The formation of the Wolffian body. 
 
 (10) The involution of the germinal epithelium to 
 form the duct of Miiller. 
 
 (11) The appearance of the primitive ova in the ger- 
 minal epithelium. 
 
 (12) The development of a fifth pair of arterial arches 
 and the obliteration of the second pair. 
 
 (13) The origin from the ductus venosus of the capil- 
 laries of the liver. 
 
 (14) The development of the 'canalis auricularis,' the 
 growth of the septum of the ventricles and of the auricular 
 appendages. 
 
CHAPTER VII. 
 
 THE CHANGES WHICH TAKE PLACE ON THE FIFTH DAY. 
 
 1. ON opening an egg about the middle of the fifth day, 
 the observer's attention is not arrested by any new features ; 
 but he notices that the progress of development, which was so 
 rapid during the later half of the fourth day, is being con- 
 tinued with undiminished vigour. 
 
 The allantois which on the fourth day began to project 
 from the pleuroperitoneal cavity has grown very rapidly, 
 and now stretches away from the somatic stalk far over the 
 right side of the embryo (which it will be remembered is 
 lying on its left side) in the cavity between the two amniotic 
 folds (Fig. 8, K). It is very vascular, and already serves 
 as the chief organ of respiration. 
 
 The blastoderm has spread over the whole of the yolk- 
 sac ; and the yolk is thus completely enclosed in a bag 
 whose walls, however, are excessively delicate and easily 
 torn. The vascular area extends over. about two-thirds of 
 the yolk. 
 
 The splanchnic stalk or umbilical duct has now reached 
 its greatest narrowness ; it has become a solid cord, and will 
 undergo no further change till near the time of hatching. 
 The space between it and the somatic stalk is still con- 
 siderable, though the latter is narrower than it was on the 
 fourth day. 
 
 2. The embryo remains excessively curved, so much so 
 indeed that the head and the tail are nearly in contact. 
 
 The limbs have increased, especially in length ; in each 
 a distinction is now apparent between the more cylindrical 
 
CHAP. VII.] THE LIMBS. 175 
 
 stalk and the flattened terminal expansion ; and the carti- 
 laginous precursors of the several bones have already become 
 visible. 
 
 The fore and hind limbs are still exceedingly alike, and 
 in both the stalk is already beginning to be bent about its 
 middle to form the elbow and knee respectively. 
 
 The angles of both knee and elbow are in the 
 first instance alike directed outwards and somewhat back- 
 wards. By the eighth day, however, the elbow has come 
 to look directly backwards and the knee forwards. In 
 consequence of this change, the digits of the fore limb 
 point directly forwards ; those of the hind limb directly 
 backwards. This state of things is altered by a subsequent 
 rotation of the hand and foot on the arm and leg, so that by 
 the tenth day the toes are directed straight forwards, and 
 the digits of the wing, backwards and somewhat downwards, 
 the elbow and knee almost touching each other. 
 
 While these changes are taking place, the differences 
 between wing and foot become more and more distinct. 
 The cartilages of the digits appear on the fifth day as streaks 
 in the broad flat terminal expansions, from the even curved 
 edge of which they do not project. On the sixth or seventh 
 day the three digits of the wing (the median being the 
 longest), and the four (or in some fowls five) digits of the 
 foot may be distinguished, and on the eighth or ninth day 
 these begin to project from the edge of the expanded foot 
 and wing, the substance of which, thin and more or less 
 transparent, remains for some time as a kind of web between 
 them. By the tenth day, the fore and hind extremities, save 
 for the absence of feathers and nails, are already veritable 
 wings and feet. 
 
 At an early period of development we find the following elements in the 
 avian manus, as separate masses of cartilage. 
 
 In the carpus there are four elements. Two in the proximal row which 
 remain distinct through life, viz. (i) the radiale, (2) the united intermedium 
 and ulnare. In the distal row (according to some recent observations of 
 I)r Rosenberg, Zeitschrift fur Wiss. Zoologie, 1873, p. 139, etc.) there are also 
 two elements. One of these is the united first and second carpal which we 
 may call carpal l ~ u , and the other is the united third and fourth carpal which 
 we may call carpal in ~ lv . These subsequently unite with the metacarpal 
 bones, and form with them a united ' carpo-metacarpus ' comparable with the 
 tarso-metatarsus of the avian foot. 
 
 Four metacarpals are present, viz. the first, second, third, and fourth. The 
 
176 THE FIFTH DAY. [CHAP. 
 
 first, second, and third are the usually recognized elements, and to these 
 Dr Rosenberg's investigations (loc. cit.) have added a fourth. The first, second 
 and third persist in the adult, though they become anchylosed in all recent 
 birds. They also fuse, as we have said above, with the distal, row of the carpals. 
 Phalanges belonging to the first, second, and third metacarpals are present. 
 
 There thus seem in the avian manus to be no representatives of the centrale, 
 the fifth carpal, the fifth nietacarpal and the phalanges of the fourth and fifth 
 digits. 
 
 Of the elements we have spoken of in the avian hand, the only ones which 
 require further notice are the carpal 1 " 11 , carpal UI - 1V , and the fourth ineta- 
 carpal. 
 
 The united first and second carpal first appears as a small mass of cartilage 
 close to the proximal end of the second metacarpal. In this condition it 
 persists for some time but commences finally to fuse with the first metacarpal ; 
 and at a slightly subsequent period with the second metacarpal. These rela- 
 tions with the first and second metacarpals shew without doubt that this 
 little mass of cartilage is the representative of the first and second bones of 
 the distal row of the carpus. In a still later stage carpal I ~ 11 fuses also with 
 carpal in ~ IY . Its distinct nature as a separate element in the bird's manus is 
 again shewn during ossification, when there appears for it a separate centre of 
 ossification. 
 
 Carpal m " IV appears about the same time as carpal 1 " 11 but is at its first appear- 
 ance united with metacarpals three and four ; it soon becomes separated from 
 metacarpal three, and afterwards also from metacarpal four. It subsequently 
 undergoes considerable changes of shape, and rather later fuses with carpal 1 " 11 . 
 Its true nature is again, as with carpal 1 " 11 shewn during ossification by the 
 appearance of a separate centre of ossification for it. 
 
 The fourth metacarpal is, as we have described, at first united with carpal m ~ IV , 
 but subsequently the neck connecting the two becomes constricted, and finally 
 they become completely separated from each other. The small independent 
 mass of cartilage thus formed represents the fourth metacarpal; it applies itself 
 closely to the side of the third metacarpal, though without becoming united with 
 it. It ossifies very late some time after the hatching of the chick, and after 
 ossification fuses with the third metacarpal and then in most cases disappears 
 completely. 
 
 The pes of a fowl in its early embryonic condition consists of 
 
 (t) a mass of cartilage close to the distal end of the tibia. It represents 
 (Gegenbaur) the proximal row of tarsal bones, viz. the l tibiale^ the l inter- 
 medium^ the 'fibularej and the 'centraleS This cartilage fuses in the adult 
 with the distal end of the tibia. 
 
 (2) a mass of cartilage representing the five bones of the distal row of 
 the tarsus. In the adult this unites with the metatarsus, forming a tarso- 
 metatarsus. 
 
 (3) the metatarsus. There are usually stated to be four metatarsal bones 
 present in the metatarsus of a fowl, which are anchylosed in the adult, but are 
 represented by separate rods of cartilage in the embryo. They are the distal 
 extremity of a first metatarsal, and complete second, third and fourth metatarsal 
 bones. In addition to these Dr Rosenberg (loc. cit.) has found a small oval mass 
 of cartilage representing a fifth metatarsal. Soon after its appearance this 
 becomes fused with the end of the tarsal mass of cartilage representing the 
 fifth tarsal, but later entirely atrophies. 
 
 (4) There are four phalanges present both in the embryo and the adult, a 
 number which is never exceeded in birds (except amongst some abnormal breeds 
 of fowls, e.g. the Dorking fowls) ; though one or more of the four are frequently 
 deficient. 
 
VII.] THE INVESTING MASS. 177 
 
 3. As we mentioned in the last chapter, the formation 
 of the primitive cranium commenced upon the fourth day. 
 This in its earliest stage, inasmuch as it is composed of con- 
 densed but otherwise only slightly differentiated mesoblast, 
 may be spoken of as the membranous cranium. 
 
 On the sixth day, true hyaline cartilage makes its ap- 
 pearance ; and the primitive membranous cranium gives 
 place to the primitive cartilaginous cranium. 
 
 The cartilage which is the first to appear, forms a thick 
 plate called the investing mass of Rathke (Fig. 55, iv.) t sur- 
 rounding the whole of that portion of the notochord which 
 projects in front of the foremost protovertebra. The hinder 
 
 FIG. 55. 
 
 VlEW FROM ABOVE OF THE INVESTING MASS AND OF THE TRABECUL-E ON 
 
 THE FOURTH DAT OF INCUBATION. (From Parker.) 
 
 In order to shew this, the whole of the upper portion of the head has been 
 sliced away. The cartilaginous portions of the skull are marked with the dark 
 horizontal shading. 
 cv. i. cerebral vesicles (sliced off), e. eye. nc. notochord. iv. investing mass. 
 
 9. foramen for the exit of the ninth nerve, cl. cochlea, hsc. horizontal 
 
 semicircular canal, q. quadrate. 5. notch for the passage of the fifth nerve. 
 
 Ig. expanded anterior end of the investing mass. pts. pituitary space. 
 
 tr. trabeculse. The reference line tr. has been accidentally made to end a 
 
 little short of the cartilage. 
 
 E. 12 
 
178 THE FIFTH DAY. [CHAP. 
 
 portion of this investing mass sends upwards along the sides 
 of the brain two lateral projections or wings, which enclose 
 the rudiments of the internal ear. In the chick the portions 
 which thus inclose the auditory sacs seem never to be at any 
 time separate from the remainder of the investing mass. At 
 the front end of the notochord the cartilaginous investing 
 mass divides into two horizontal branches in the form of two 
 cartilaginous rods called the trabeculce (Fig. 55, tr.) y which 
 passing forward (in a somewhat different plane from the 
 investing mass), meet again in front, and so enclose a space 
 called the pituitary space pts, into which the infundibulum 
 extends downward. In front of this junction, the trabeculse 
 expand into a somewhat broad plate (subsequently developed 
 into the ethmoid and nasal cartilages), which ends in two 
 horns in the interior of the fronto-nasal process. 
 
 The front end of the notochord probably defines the 
 anterior boundary of the basi-occipital. At first it extends 
 quite up to the pituitary space and the starting-point of the 
 trabeculse. Subsequently, however, there takes place be- 
 tween it and the pituitary space a growth of cartilage in 
 which the ossification for the basi- sphenoid takes place. 
 
 The lateral projections at the hinder end of the investing 
 mass grow up behind, and completely enclose that part of 
 the neural canal from which the medulla oblongata is de- 
 veloped, and in it ossifications arise to form the occipital 
 bones and the bones which invest the auditory labyrinth. 
 
 It is important to notice that the only segment of the 
 skull, which primarily forms a cartilaginous roof to any part 
 of the brain, is the occipital segment. The roof of the re- 
 mainder of the skull is formed by membrane-bones. 
 
 For the histological differences observable in the develop- 
 ment of cartilage and membrane bones, we must refer 
 the reader to treatises on histology; for our purpose it is 
 sufficient to say that a membrane-bone is one which is not 
 preformed in cartilage, while a cartilage-bone is one in which 
 the ossification takes place in a bed of cartilage, which fills 
 the place subsequently occupied by the bone. 
 
 The trabeculaa together with the cartilage between the 
 pituitary space and the end of the notochord give rise to the 
 sphenoid bone, while in the cartilage in front of the trabeculae 
 the ethmoid and nasal bones are formed. 
 
VII.] THE VISCERAL ARCHES. 179 
 
 From the study of the development of the skull, especially in some of tl>e 
 lower vertebrates, Mr Parker and Professor Huxley have shewn, that the 
 trabeculse are developed independently of the investing mass, and that their 
 subsequent connection with it is due to a secondary process. Professor Huxley 
 is of opinion that they are to be regarded as the remains of a pair of visceral 
 arches, corresponding with the other five pah's of arches which we find developed 
 in the chick. The stage in which they exist as simple visceral arches with a 
 core of undifferentiated mesoblast is not seen in the chick. They first attract 
 notice when they become cartilaginous rods. 
 
 The ordinary visceral arches are, as we have seen, suffici- 
 ently .obvious, while as yet their mesoblast is quite undiffer- 
 entiated ; but in them, as in the trabeculae, rods of cartilage 
 are subsequently developed and begin to make their appear- 
 ance about the fifth day. 
 
 The first arch, it will be remembered, budded off a 
 process called the superior maxillary process. The whole 
 arch, therefore, comes to consist of two parts, viz. a superior 
 and an inferior maxillary process; in each of these, carti- 
 laginous rods are developed. In the superior maxillary 
 process, the rod does not appear till the fifth day. It is 
 called from its subsequent fate, the ptery go-palatine rod, and 
 consists of a pterygoid and of a palatine part. In the 
 inferior maxillary process two developments of cartilage take 
 place ; one which forms the quadrate in the upper or prox- 
 imal portion close to the origin of the superior maxillary 
 process, a second in the lower or distal portion, which goes 
 by the name of Meckel's cartilage. 
 
 Cartilaginous rods are also formed in the second and third 
 arches. These, which give rise to the hyoids and branchials 
 respectively, quickly come to lie within the first arch, but 
 do not form a conspicuous portion of the skeleton of the 
 face. 
 
 4. Closely connected with the development of the skull 
 is the formation of the parts of the face. 
 
 After the appearance of the nasal grooves, on the fourth 
 day the mouth (Fig. 56 Jlf.) appears as a deep depression 
 inclosed by five processes. Its lower border is entirely 
 formed by the two inferior maxillary processes (Fig. 56, F.I), 
 at its sides lie the two superior maxillary processes S. M, 
 while above it is bounded by the fronto-nasal process nf. 
 
 After a while the outer angles of the fronto-nasal process, 
 enclosing the termination of the ethmovomerine plate, pro- 
 ject somewhat outwards on each side, giving the end of the 
 
 19 9 
 
180 THE FIFTH DAY. [CHAP. 
 
 FIG. 56. 
 
 B 
 
 A. HEAD OF AN EMBRYO CHICK OF THE FOURTH DAT VIEWED FROM BELOW 
 AS AN OPAQUE OBJECT. (Chromic acid preparation.) 
 
 CH. cerebral hemispheres. FB. vesicle of the third ventricle. Op. eyeball. 
 nf. naso-frontal process. M. cavity of mouth. S. M. superior maxillary 
 process of F. r, the first visceral fold (inferior maxillary process). F. 2, F. 3, 
 second and third visceral folds. N. nasal pit. 
 
 In order to gain the view here given the neck was cut across between the 
 third and fourth visceral folds. In the section e thus made, are Been the 
 alimentary canal al with its collapsed walla, the neural canal ra.c., the noto- 
 chord cA., the dorsal aorta AO., and the vertebral veins V. 
 
 The incision has been carried just below the upper limit of the pleuroperi- 
 toneal cavity, consequently a portion of the somatopleure appears at the angle 
 between the two third visceral folds. Almost embraced by the piece of somato- 
 pleure is seen the end of the bulbus arteriosus Ao. 
 
 In the drawing the nasal groove has been rather exaggerated in its upper 
 part. On the other hand the lower part of the groove, where it runs between 
 the superior maxillary process S. M. and the broad naso-frontal process, was in 
 this particular embryo extremely shallow and indeed hardly visible. Hence 
 the end of the superior maxillary process seems to join the inner and not, as 
 described in the text, the outer margin of the nasal groove. A few hours later 
 the separation of the two would have been very visible. 
 
 B. The same seen sideways, to shew the visceral folds. Letters as before. 
 
 process a rather bilobed appearance. These projecting portions 
 of the fronto-nasal process form on each side the inner 
 margins of the rapidly deepening nasal grooves, and are 
 sometimes spoken of as the inner nasal processes. The outer 
 margin of each nasal groove is raised up into a projection 
 frequently spoken of as the outer nasal process which runs 
 downwards to join the superior maxillary process, from which, 
 
VII.] THE NASAL LABYRINTH. 181 
 
 however, it is separated by a shallow depression. This de- 
 pression, which runs nearly horizontally outwards towards the 
 eyeball, is, according to Coste and Kolliker, subsequently con- 
 verted into the lachrymal duct. 
 
 On the fifth day, the inner nasal processes or lower and 
 outer corners of the fronto-nasal process arching over, unite 
 on each side with the superior maxillary processes. (Com- 
 pare Fig. 57, which, however, is a view of the head of a 
 chick , of the sixth day.) In this way each nasal groove is 
 converted into a canal, which leads from the nasal pit above, 
 into the cavity of the mouth below, and places the two in 
 direct communication. This canal, whose lining consists of 
 epiblast, is the rudiment of the nasal labyrinth. 
 
 FIG. 57. 
 
 HEAD OF A CHICK AT THE SIXTH DAY FROM BELOW. (Copied from Huxley's 
 Elements of Comparative Anatomy.) 
 
 la. cerebral vesicles, a. eye, in which the remains of the choroid slit can still be 
 seen. g. nasal pits. k. fronto-nasal process. I. superior maxillary process. 
 I. inferior maxillary process or first visceral arch. 2. second visceral arch. 
 x. first visceral cleft between the first and second visceral arches. 
 The cavity of the mouth is seen enclosed by the fronto-nasal process, the 
 superior maxillary processes and the first pair of visceral arches. At the back 
 of it is seen the opening leading into the throat. The nasal grooves leading 
 from the nasal pits to the mouth are already closed over and converted into 
 canals. . 
 
 By the seventh day (Fig. 58), not only is the union of 
 the superior maxillary and fronto-nasal processes completed, 
 and the upper boundary of the mouth thus definitely con- 
 
182 THE FIFTH DAY. [CHAP. 
 
 stituted, but these parts begin to grow rapidly forward, thus 
 deepening the mouth, and giving rise to the appearance of a 
 nose or beak (Fig. 58), which, though yet blunt, is still 
 distinct. The whole of the lower boundary of the buccal 
 cavity is formed by the inferior maxillary processes. 
 
 FIG. 58. 
 
 HEAD OF A CHICK OF THE SEVENTH DAT FROM BELOW. (Copied from Huxley's 
 Elements of Comparative Anatomy.) 
 
 I a. cerebral vesicles, a. eye. g. nasal pits. Tc. fronto-nasal process. I. superior 
 maxillary process. I. first visceral arch. 2. second visceral arch. x. first 
 visceral cleft. 
 
 The external opening of the mouth has become much constricted, but it is 
 fitill enclosed by the fronto-nasal process and superior maxillary processes above, 
 and by the inferior maxillary process (first pair of visceral arches) below. 
 
 The superior maxillary processes have united with the fronto-nasal process, 
 along the whole length of the latter, with the exceptio" of a small space in 
 iront, where a narrow angular opening is left between the two. 
 
 As we have before mentioned, the ethmovomerine car- 
 tilage is developed in the fronto-nasal process, the pterygo- 
 palatine bar in the superior maxillary process, Meckel's 
 cartilage and the quadrate in the inferior maxillary process ; 
 the other bones which form the boundaries of the mouth in 
 the adult are developed later after all external trace of these 
 parts as separate processes has disappeared. For an account 
 of their formation, however, we must refer the reader to the 
 chapter upon the development of the skull. 
 
 At first the mouth is a simple cavity into which the 
 nasal canals open directly. When however the various 
 
VII.] THE MOUTH. 18S 
 
 processes unite together to form the upper boundary of the 
 mouth, each superior maxillary process sends inwards a lateral 
 bud. These buds become flattened and form horizontal plates 
 which stretch more and more inward towards the middle line. 
 There they finally meet, and by their union, which is effected 
 first in front, and thence extends backwards, they con- 
 stitute a horizontal plate, stretching right across the mouth, 
 and dividing it into two cavities an upper and a lower one. 
 In the front of the mouth their union is quite complete, 
 so that here there is no communication between the two 
 cavities. Behind, however, the partition is not a complete 
 one ; so that the two divisions of the buccal cavity com- 
 municate at the back of the mouth. The external opening 
 of the mouth passes into the lower of these two cavities, 
 which may therefore be called the mouth proper. Into the 
 upper chamber the nasal ducts open; it maybe called the 
 respiratory chamber and forms the commencement of the 
 chamber of the nose. In birds generally the upper nasal 
 cavity becomes subsequently divided by a median partition 
 into two chambers, which communicate with the back of the 
 mouth by separate apertures. The original openings of the 
 nasal pits remain as the nostrils. 
 
 5. One important occurrence of the fifth day is the 
 appearance of the anus, which is formed very much in the 
 same way as the mouth. 
 
 Beneath the tail an involution of the epiblast takes place 
 towards the cloaca. At this point the wall of the cloaca, 
 which has here taken no share in the cleavage of the meso- 
 blast, becomes thinner, and is finally perforated. An orifice 
 thus places the cloaca in communication with the exterior, 
 and constitutes the anus. 
 
 6. On this day also important changes take place in 
 the spinal cord ; and a brief history of the development of 
 this organ may fitly be introduced here. 
 
 At the beginning of the third day, the cavity of the 
 neural canal (Fig. 41) is still of considerable width, and when 
 examined in vertical section its sides may be seen to be nearly 
 parallel, though perhaps approximating to each other more 
 below than above. 
 
 We say below and above, because a vertical section is naturally examined 
 with its dorsal side uppermost. In the ordinary terminology of the spinal cord, 
 
184 THE FIFTH DAY. [CHAP. 
 
 above would be posterior and below anterior. These latter terms it will be 
 henceforward most convenient to adopt. 
 
 The exact shape/however, varies according to the region 
 of the body from which the section is taken. 
 
 The epiblast walls are at this time composed of radiately 
 arranged columnar cells. The cells are much elongated, but 
 s-omewhat irregular; and it is very difficult in sections to 
 make out their individual boundaries. They contain granu- 
 lar oval nuclei in which a nucleolus can almost always be 
 seen. The walls of the canal are both anteriorly and pos- 
 teriorly considerably thinner than in the middle. 
 
 Towards the end of the third day, changes take place in 
 the shape of the cavity. In the lumbar region its vertical 
 section becomes more elongated, and at the same time very 
 narrow in the middle while expanded at each end into a some- 
 what bulbous enlargement, producing an hour-glass appear- 
 ance (Fig. 44). Its walls however still preserve the same 
 histological characters as before. 
 
 On the fourth day (Fig. 47) coincidently with the appear- 
 ance of the spinal nerves, important changes may be observed 
 in the hitherto undifferentiated epiblastic walls. 
 
 In the anterior region of the cord, the external portions of 
 the epiblast become modified into grey matter, forming an 
 anterior grey column, which in turn is covered superficially by 
 a mass of white matter forming an anterior white column. 
 The internal portions of the epiblast remain as the epithe- 
 lium lining the spinal canal. Both columns are formed at 
 the point of entrance of the anterior nerve-roots ; and these 
 may easily be traced through the white into the grey matter. 
 
 The grey column is composed of numerous small nuclei, 
 each of which appears to be surrounded by a definite mass of 
 protoplasm, though the boundaries of the protoplasm belong- 
 ing to each nucleus can only occasionally be made out. 
 The nuclei lie in the meshes of a network of fibres continu- 
 ous with the fibres of the nerve-root, and passing through 
 the mass of grey matter in two directions : (1) round the 
 anterior end of the spinal canal, immediately outside its 
 epithelium and so to the grey matter on the opposite side, 
 forming in this way an anterior commissure through which a 
 decussation of the fibres from the opposite sides is effected : 
 (2) upwards along the outside of the lateral walls of the canal. 
 
VII.] THE SPINAL CORD. 185 
 
 The posterior roots of the spinal nerves enter the cord 
 near its posterior surface, and at this point the posterior grey 
 columns are formed in a similar way to the anterior. In 
 some cases .also the rudiment of a posterior white column 
 may be seen at the junction of the nerve with the epiblast of 
 the canal. The fibres of the posterior root cannot be traced 
 so far into the cord as those of the anterior root. 
 
 The grey matter of the cord seems undoubtedly to be formed by a meta- 
 morphosis of the external cells of the epiblast of the neural tube, and is 
 directly continuous with the epithelium ; there being no strong line of demarcation 
 between them. Whether the fibres which traverse it, and which seem to l>e 
 partly nervous and partly connective tissue in their nature, are derived from 
 inesoblast or epiblast our observations have not enabled us to determine. 
 
 The white matter which caps the grey mass, and which forms the com- 
 mencement of the anterior white column, is a peculiar tissue. It consists of a 
 network of fibres somewhat resembling the connective tissue network of the 
 white matter of the adult cord, to which it has a further likeness in not being 
 easily stained by carmine. The fibres of which it is composed have a general 
 tendency to be disposed in radiating septa, a peculiarity which is especially 
 noticeable with low powers. Along the fibres and more especially in the septa, 
 numerous highly refracting granules are embedded, and in the meshes pale 
 spherical nuclei with nucleoli are to be seen. The boundary between the white 
 and grey matter is very sharply defined, and we have always failed to trace the 
 fibres of which we are speaking into the fibres present in the grey matter, though 
 Lockhart Clarke (Phil. Trans. 1862) asserts that they are continuous. Nor can 
 the fibres of the nerve-roots be seen to come into connection with these same 
 fibres. It has generally been assumed that the white matter like the grey is 
 derived from the epiblast : this does not however appear ever to have been 
 clearly proved, while the peculiarities of the tissue, and the fact that it first 
 appears at the origin of the spinal nerves, might seem to indicate that it is 
 directly derived from the mesoblast surrounding the cord ; a view which we are 
 inclined to accept. . 
 
 On the fourth day there is no trace of either an anterior 
 or a posterior fissure, and in the lumbar region the shape of 
 the spinal canal is not very different from what it was on the 
 third day. It appears in sections as a narrow slit dilated 
 somewhat at either end (Fig. 47). The epithelium surround- 
 ing the slit is still very thin, especially above and below, but 
 at the anterior end forms a somewhat arched projection 
 with the convex surface turned downwards. 
 
 On the fifth and sixth days important changes take place. 
 
 By the great increase of the grey matter, which now 
 comes to form the chief mass of the cord, the epithelium is 
 reduced to a thin layer of cells immediately surrounding the 
 canal. 
 
 In the dorsal region, the side walls of the laterally com- 
 
186 THE FIFTH DAY. [CHAP. 
 
 pressed canal come into absolute contact in the middle. So 
 that sections no longer shew an hour-glass cavity, but two 
 more or less elliptical cavities, representing the former term- 
 inal enlargements, one anterior and one posterior, separated 
 by a neck in which the epithelium of the one side is closely 
 applied to that of the other. In other words, the original 
 single canal has been divided longitudinally into an anterior 
 and posterior canal. Of these the anterior will alone remain 
 as the permanent central canal of the spinal cord. In the 
 lumbar region this division has as yet not taken place. 
 
 The anterior white columns have very much increased 
 in quantity ; the posterior white columns have also become 
 distinct, and the two form together a thick covering for the 
 grey matter. The two columns of each side are continuous 
 with each other, but their line of junction is clearly marked ; 
 and on the sixth day there may be seen at this spot a small 
 mass of white matter, differing somewhat from the rest in 
 appearance, which perhaps may be looked upon as the first 
 commencement of the lateral column. The columns of the 
 one side are not continuous with those of the other either 
 posteriorly or anteriorly. In other words, there are as yet 
 no white commissures. 
 
 The anterior ends of the cord on each side of the middle 
 line have commenced to grow downwards. These outgrowths, 
 in which both the white and the grey matter take part, 
 have an important function. They enclose between them a 
 somewhat linear space : the commencement of the anterior 
 fissure. This, which is at first not very deep and rather wide, 
 may be noticed already on the fifth day (L. Clarke) and on 
 the sixth day is very clearly marked. 
 
 Corresponding with these grosser changes, certain histo- 
 logical features make their appearance. Between the an- 
 terior and posterior parts into which the grey matter is 
 divided on each side, or, as we may now call them, the 
 anterior and posterior cornua, there is found a rather lighter 
 band of grey matter in which the nuclei are somewhat more 
 scattered. The anterior cornu exhibits a further division 
 into an outer and upper part, and a lower and inner part, 
 in both of which the nuclei are more numerous than in the 
 intervening mass. The posterior cornu is of considerably 
 darker colour than the anterior, the difference being due to 
 
VII.] THE POSTERIOR FISSURE. 187 
 
 the greater number of nuclei present in the former. The 
 outlines of the cells are more clearly marked and somewhat 
 more angular in shape than they were on the fourth day. 
 
 The distinctions between the several parts of the grey matter are chiefly 
 brought about by variations in the number of nuclei in a given area. Throughout 
 the cord fibres of the grey matter seem to be continuous with the epithelium of 
 the neural canal, but this is much more strongly marked in the posterior than in 
 the anterior region. In the posterior region also, it is still much more difficult 
 to trace the roots of the nerves than in the anterior. 
 
 Of the three columns into which the white matter on 
 each side is divided, the anterior column differs from the 
 posterior in being thicker and also in having wider meshes 
 and fewer granules. The lateral column is the most granular 
 of all and very conspicuous. The minute structure of the 
 white matter remains about the same as on the fourth day. 
 
 Meanwhile an alteration is taking place in the external 
 outline of the cord. From being, as on the fourth and fifth 
 days, oval in section, it becomes, chiefly through the increase 
 of the white matter, much more nearly circular. 
 
 On the seventh day the most important event is the 
 formation of the posterior fissure. 
 
 This is brought about by the absorption of the roof of 
 the posterior of the two parts into which the neural canal 
 has become divided. 
 
 Between the posterior horns of the cord, the epithelium 
 forming the roof of the, so to speak, posterior canal is 
 along the middle line covered neither by grey nor by 
 white matter, and on the seventh day is partially absorbed, 
 thus transforming the canal into a wedge-shaped fissure, 
 whose mouth however is seen in section to be partially 
 closed by a triangular clump of elongated cells (Fig. 59 c.). 
 Below this mass of cells the fissure is open. It is separated 
 from the 'true spinal canal ' by a very narrow space along 
 which the side walls have coalesced. In the lumbar and 
 sacral regions the two still communicate. 
 
 We thus find, as was first pointed out by Lockhart Clarke, 
 that the anterior and posterior fissures of the spinal cord 
 are, morphologically speaking, entirely different. The ante- 
 rior fissure is merely the space left between two lateral 
 downward growths of the cord, while the posterior fissure is 
 part of the original neural canal separated from the rest of 
 
188 
 
 THE FIFTH DAY. 
 FIG. 59. 
 
 [CHAP, 
 
 SECTION THROUGH THE SPINAL CORD OP A SEVEN DAYS' CHICK. 
 
 p.C. w. posterior white column. Z. c. w. lateral white column, a. c. w. anterior 
 white column, p c. posterior corn u, of grey matter, consisting of very 
 small cells, a. c. anterior cornu of grey matter, with a peculiar mass of 
 very large cells, ep. epithelium lining the original medullary canal, p.f. pos- 
 terior fissure. The posterior fissure is chiefly formed by the upper portion 
 of the original medullary canal which becomes open above. The upper 
 portion of it is now filled with tissue (c) which is probably derived from 
 the epithelium of the medullary canal. The lower portion of the medullary 
 canal becomes the spinal canal (sp. c.) and is eventually entirely shut off 
 from posterior fissure. The communication between the spinal canal and the 
 posterior fissure is already narrowed, and if the section had been made 
 further forwards, the two would have been entirely separated from each 
 other. 
 
 a. /. anterior fissure. This is formed in an entirely different manner from 
 the posterior fissure. It is produced by the anterior column of white, 
 and the anterior cornu of grey matter, growing downwards and leaving 
 between them a fissure. It is at this time filled up with connective 
 tissue. 
 
 a. g. c. anterior grey commissure, c. tissue filling up the end of the posterior 
 fissure, sp. c. spinal canal. Only the right half of the cord is represented 
 in the figure. The section passes through the cord between the entrance 
 of two spinal nerves. The angular form of the cells of the cord has not 
 been done justice to by the engraver. 
 
VII.] THE WHITE COLUMNS. 189 
 
 the cavity (which goes to form the true spinal canal) by 
 a median coalescence of the side walls. 
 
 The lateral white columns have on the seventh day 
 increased in size and become less granular, and the lines of 
 junction between them and the anterior columns have now to 
 be arbitrarily selected. The posterior white columns are 
 still much thinner and more granular than the anterior. The 
 nuclei of the white matter are more numerous than before. 
 
 Some of the septa of the white matter can now be traced in the one 
 direction into the grey matter, and in the other direction into the connective 
 tissue around the cord. Whether these are nerve-fibres which have separated 
 irom the remainder of the fibres, to enter the cord at a different point, or are 
 merely trabecuiae of connective tissue, cannot be absolutely determined. The 
 latter view however seems most probable. In the grey matter, the anterior 
 and posterior divisions are better distinguished than at an earlier date. In 
 particular the nuclei of the cells of the posterior division are both smaller and 
 more numerous than those of the anterior. Some of the fibres from the 
 posterior root, after entering the grey matter, quickly pass out again into the 
 posterior column of the white matter. 
 
 In the anterior division of the grey matter, near the entrance of the anterior 
 roots, there is a peculiar and well-marked mass of somewhat triangular cells, 
 with large and distinct nuclei, more deeply stained with carmine than the 
 remainder of the grey matter. This mass of cells is present in the lumbar and 
 sacral regions, but is deficient or very inconspicuous in the dorsal portion of the 
 cord. The nuclei of the whole anterior region of the grey matter have increased 
 in size, and the cells to which they belong (when clearly visible) are usually 
 iound to be angular. 
 
 Around the true spinal canal, the line of separation between the epithelium 
 and the grey matter is sharply defined, but elsewhere is very indistinct. 
 
 By the end of the seventh day, the following important 
 parts of the cord have been definitely established : 
 
 (1) The anterior and posterior fissures. 
 
 (2) The anterior and posterior horns of grey matter. 
 
 (3) The anterior, posterior and lateral columns of white 
 matter. 
 
 (4) The spinal canal. 
 
 As yet, however, the grey mass of the two sides of the 
 cord only communicate by the anterior grey commissure, and 
 the white columns of opposite sides do not communicate 
 at all. The grey matter, moreover, still far preponderates 
 over the white matter in quantity. 
 
 By the ninth day the posterior fissure is fully formed, 
 and the posterior grey commissure has also appeared. 
 
 In the centre of the sacral enlargement this commissure 
 is absent, and the posterior columns at a later period separate 
 
190 THE FIFTH DAY. [CHAP. 
 
 widely and form the ' sinus rhomboidalis,' which is not, as 
 has been sometimes stated, the remains of the primitive 
 'sinus rhomboidalis' visible during the second day. 
 
 The anterior white columns have much increased on this 
 day, and now form the sides of the already deep anterior 
 fissure. The anterior white commissure does not however 
 appear till somewhat later. 
 
 7. The fifth day may perhaps be taken as marking a 
 most important epoch in the history of the heart. The 
 changes which take place on that and on the sixth day, 
 added to those previously undergone, transform the simple 
 tube of the early days of incubation into an almost com- 
 pletely formed heart. 
 
 The venous end of the heart, though still lying somewhat 
 to the left and above, is now placed as far forwards as the 
 arterial end, the whole organ appearing to be drawn together. 
 The ventricular septum is complete. 
 
 The apex of the ventricles becomes more and more 
 pointed. In the auricular portion a small longitudinal fold 
 appears as the rudiment of the auricular septum, while in 
 the canalis auricularis, which is now at its greatest length, 
 there is also to be seen a commencing transverse partition 
 tending to separate the cavity of the auricles from those of 
 the ventricles. 
 
 About the 106th hour, a septum begins to make its 
 appearance in the bulbus arteriosus in the forrn of a longitu- 
 dinal fold, which according to Dr Tonge (Proc. of Royal Soc. 
 1868) starts, not (as Von Baer thought) at the end of the 
 bulbus nearest to, but at that furthest removed from, the 
 heart. It takes origin from the wall of the bulbus between 
 the fifth and fourth pairs of arches and grows downwards in 
 such a manner as to divide the bulbus into two channels, one 
 of which leads from the heart to the fourth and third pair of 
 arches and the other to the fifth pair. The free edge of the 
 septum is somewhat V-shaped, so that its two legs as it were 
 project downwards towards the heart, further than its central 
 portion ; and this shape of the free edge is maintained 
 during the whole period of its growth. Its course downwards 
 is not straight but spiral, and thus the two channels into 
 which it divides the bulbus arteriosus, wind spirally the one 
 over the other. The existence of the septum can only be 
 
VII.] THE SEMILUNAR VALVES. 191 
 
 ascertained at this stage by dissection or by sections, there 
 being as yet no external signs of the division. 
 
 At the time when the septum is first formed, the opening 
 of the bulbus arteriosus into the ventricles is narrow or slit- 
 like, apparently in order to prevent the flow of the blood 
 back into the heart. Soon after the appearance of the septum, 
 however, semilunar valves (Tonge, loc. cit.) are developed 
 from the wall of that portion of the bulbus which lies between 
 the free edge of the septum and the cavity of the ventricles. 
 
 These arise as six solid outgrowths of the wall arranged in 
 pairs, an anterior, an inner, and an outer pair, one valve of 
 each pair belonging to the one and the other to the other of 
 the two main divisions of the bulbus which are now being 
 established. 
 
 The anterior and the inner pairs of valves are the first to 
 appear : the former as two small solid prominences separated 
 from each other by a narrow groove ; the latter as a single 
 shallow ridge, in the centre of which is a prominence indi- 
 cating the point where the ridge will subsequently become 
 divided into two. The outer pair of valves appear opposite 
 each other, at a considerably later period, between the ends 
 of the other pair of valves on each side. 
 
 As the septum grows downwards towards the heart, it 
 finally reaches the position of these valves. One of its legs 
 then passes between the two anterior pair of valves, and the 
 other unites with the prominence on the inner valve-ridge. 
 At the same time the growth of all the parts causes the 
 valves to appear to approach the heart and thus to be placed 
 quite at the top of the ventricular cavities. The free edge of 
 the septum of the bulbus now fuses with the ventricular 
 septum, and thus the division of the bulbus into two separate 
 channels, each provided with three valves, and each com- 
 municating with a separate side of the heart, is complete, 
 the position of the valves not being very different from what 
 it is in the adult heart. 
 
 That division of the bulbus which opens into the fifth pair 
 of arches is the one which communicates with the right 
 ventricle, while that which opens into the third and fourth 
 pairs communicates with the left ventricle. The former 
 becomes the pulmonary artery, the latter the commencement 
 of the systemic aorta. 
 
192 
 
 THE FIFTH DAY. 
 
 [CHAP. 
 
 The external constriction actually dividing the bulbus 
 into two vessels, does not begin to appear till the septum has 
 extended some way back towards the heart. 
 
 The semilunar valves become pocketed at a period con- 
 siderably la'ter than their first formation (from the 147th to 
 the 165th hour) in the order of their appearance. 
 
 8. Towards the end of the fifth and in the course of the 
 sixth day further important changes take place in the heart. 
 
 The venous end with its two very conspicuous auricular 
 appendages, comes to be situated more above (dorsal to) the 
 arterial end, though it still turns rather towards the left. 
 The venous portion of the heart undergoes on the sixth day 
 or even near to the end of the fifth, such a development of the 
 muscular fibres of its walls, that the canalis auricularis becomes 
 almost entirely concealed. The point of the heart is now 
 
 TWO VIEWS OF THE HEART OF A CHICK UPON THE FlFTH DAY OF INCUBATION, 
 
 A from the ventral, B from the dorsal side. 
 
 /. a. left auricular appendage, r. a. right auricular appendage, r. x. right 
 ventricle, l.v. left ventricle. 6. bulbus arteriosus. 
 
VII.] THE BULBUS ARTERIOSUS. 193 
 
 directed nearly backwards (i.e. towards the tail), but also a 
 little downwards. 
 
 An alteration takes place during the sixth day in the 
 relative position of the parts of the ventricular division of 
 the heart. The right ventricle is now turned towards the 
 abdominal surface, and also winds to a certain extent round 
 the left ventricle. It will be remembered that on the fourth 
 day the right ventricle was placed above (dorsal to) the left. 
 
 The right ventricle is now also the smaller of the two, 
 and the constriction which divides it from the left ventricle 
 does not extend to the apex of the heart (Fig. 60) It has, 
 however, a very marked bulge towards the right. 
 
 At first the bulbus arteriosus appeared to come off 
 chiefly from the left ventricle ; during the fifth day, and 
 still more on the sixth, it appears to come from the right 
 chamber. This is caused by the canal from the right ven- 
 tricle into the bulbus arteriosus passing towards the left, 
 and on the ventral side, so as entirely to conceal the origin of 
 the canal from the left chamber of the heart. On the seventh 
 day the bulbus arteriosus appears to come less markedly from 
 the right side of the heart. 
 
 All these changes, however, of position of the bulbus 
 arteriosus only affect it externally; during the whole time 
 the two chambers of the heart open respectively into the two 
 divisions of the bulbus arteriosus. The swelling of the 
 
 FIG. 61. 
 T.O, 
 
 T.V 
 
 HEART OF A CHICK UPON THE SIXTH DAY OF INCUBATION, FBOM THE 
 VENTRAL SURFACE. 
 
 I. a. left auricular appendage, r.a. right auricular appendage, r.v. right ventricle, 
 I. v. left ventricle, b. bulbus arteriosus. 
 
 E. 13 
 
194 THE FIFTH DAY. [CHAP. 
 
 bulbus is much lestf marked on the seventh day than it was 
 before. 
 
 At the end of the sixth day, and even on the fifth day 
 (Figs. 60, 61), the appearance of the heart itself, without 
 reference to the vessels which come from it, is not very 
 dissimilar from that which it presents when adult. 
 
 The original curvature to the right now forms the apex 
 of the ventricles, and the two auricular appendages are 
 placed at the anterior extremity of the heart. 
 
 The most noticeable difference (in the ventral view) is 
 the still externally undivided condition of the bulbus arte- 
 riosus. 
 
 About the sixth or, perhaps, even on the fifth day, the 
 pericardium, according to Von Baer, makes its appearance. 
 Its mode of formation is not exactly known, but it probably 
 takes origin from folds of the lining of the thoracic cavity 
 which meet and coalesce. 
 
 9. The subsequent changes which the heart undergoes 
 are concerned more with its internal structure than with its 
 external shape. Indeed, during the next three days, viz. the 
 eighth, ninth, and tenth, the external form of the heart re- 
 mains nearly unaltered. 
 
 In the auricular portion however, the septum which com- 
 menced on the fifth day becomes now more conspicuous. It 
 is placed vertically, and arises from the ventral wall ; com- 
 mencing at the canalis auricularis and proceeding backwards, 
 it does not as yet reach the opening into the sinus venosus. 
 
 The blood from the sinus, or, as we may call it, the 
 inferior vena cava, enters the heart obliquely from the right, 
 so that it has a tendency to flow towards the left auricle of the 
 heart, which is at this time the larger of the two. 
 
 The valves between the ventricles and auricles are now 
 well developed, and it is about this time that the division of 
 the bulbus arteriosus into the aorta and pulmonary artery 
 becomes visible on the exterior. 
 
 By the eleventh or thirteenth day the right auricle has 
 become as large as the left, and the auricular septum much 
 more complete, though there is still a small opening, the 
 foramen ovale, by which the two cavities communicate 
 with each other. Through this foramen the greater part of 
 the blood of the vena cava inferior, which is now joined just 
 
VII.] THE EUSTACHIAN VALVE. 195 
 
 at its entrance into the heart by the right vena cava superior, 
 is directed into the left auricle. The left vena cava superior 
 enters the right auricle independently ; between it and the 
 inferior vena cava is a small valve which directs its blood 
 entirely into the right auricle. 
 
 On the sixteenth day the right vena cava superior, when 
 viewed from the exterior, still appears to join the 'inferior 
 vena cava before entering the heart ; from the interior how- 
 ever the two can now be seen to be separated by a valve. 
 This valve, called the 'Eustachian valve/ extends to the 
 opening of the left vena cava superior, and into it the valve 
 which in the earlier stage separated the left superior and 
 inferior venae cava3 has apparently become merged. There is 
 also on. the left side of the opening of the inferior cava a 
 membrane, stretching over the foramen ovale, and serving as 
 a valve for that orifice. The blood from the inferior cava 
 still passes chiefly into the left auricle through the foramen 
 ovale ; while the blood from the other two venae cava3 now 
 falls into the right auricle, being prevented from entering 
 the left chamber by the Eustachian valve. 
 
 Hence, since at this period also the blood from the left 
 ventricle passes to a great extent to the anterior portion 
 of the body, there is a species of double-circulation going 
 on. The greater part of the blood from the allantois entering 
 the left auricle from the inferior vena cava passes into the 
 left ventricle and is thence sent chiefly to the head and 
 anterior extremities ; from these it is brought back through 
 the right auricle to the right ventricle, from whence it is 
 returned along the aorta to the allantois. 
 
 From the seventeenth to the nineteenth day, the right 
 auricle becomes larger than the left. The large Eustachian 
 valve still prevents the blood from the superior cavae from 
 entering the left auricle, while it conducts the blood from the 
 inferior vena cava into that chamber through the foramen 
 ovale. The entrance of the inferior vena cava is however 
 further removed than it was from the foramen ovale, and the 
 increased flow of blood from the lungs prevents all the blood 
 )f the inferior cava from entering into the left auricle. At 
 the same time the valve of the foramen ovale prevents the 
 blood in the left auricle from entering the right auricle. 
 
 During the period from the seventh day onwards, the 
 
 13-2 
 
196 THE FIFTH DAY. [CHAP. 
 
 apex of the heart becomes more marked; the arterial roots 
 are more entirely separated and the various septa completed, 
 so that when the foramen ovale is closed and the blood of 
 the inferior vena cava thereby entirely confined to the 
 right auricle, the heart has practically acquired its adult con- 
 dition. 
 
 10. The fifth day may also be taken as marking the 
 epoch at which histological differentiation first becomes 
 distinctly established. 
 
 It is of course true that long before this date, even from 
 the earliest hours, the cells in each of the three fundamental 
 layers have ceased to be everywhere alike. Nevertheless the 
 changes undergone by the several cells have been few and slight. 
 The cells of epiblastic origin, both those going to form the 
 epidermis and those included in the neural involution, are up 
 to this time simple more or less columnar cells ; they may be 
 seen here elongated, there oval, and in another spot spheroidal ; 
 here closely packed with scanty protoplasm, there scattered 
 with each nucleus well surrounded by cell-substance ; but 
 wherever they are found they may still be recognized as cells 
 of a distinctly epithelial character. So also with the cells 
 of hypoblastic origin, whether simply lining the alimentary 
 canal or taking part in the formation of the compound 
 glands. Even in the mesoblast, which undergoes far more 
 changes than either of the other layers, not only increasing 
 more rapidly in bulk but also serving as the mother tissue 
 for a far greater number of organs, the alterations in the 
 individual cells are, till near upon the fifth day, insignificant. 
 Up to this time, the mesoblast may be spoken of as consisting 
 of little more than indifferent tissue : of nuclei imbedded in 
 a protoplasmic cell-substance. In one spot the nuclei are 
 closely packed together, and the cell-substance scanty and 
 compact ; at another the nuclei are scattered about with 
 spindle-shaped masses of protoplasm attached to each, and 
 there is a large development either of intercellular spaces or 
 of intracellular vacuoles, filled with clear fluid. The proto- 
 plasm differs in various places, chiefly in being more or less 
 granular, and less or more transparent, having as yet under- 
 gone but slight chemical transformation. Up to this epoch 
 (with the exception of the early differentiated blood), there 
 are no distinct tissues, and the rudiments of the various 
 
VII.] THE EPIBLAST. 197 
 
 organs are simply marked out by greater or less condensation 
 of the simple mesoblastic substance. 
 
 From the fifth day onwards, however, histological differ- 
 entiation takes place rapidly ; and it soon becomes possible 
 to speak of this or that part as being composed of muscular, 
 or cartilaginous, or connective &c. tissue. It is not within 
 the scope of the present work to treat in detail of these histo- 
 genetic changes, for information concerning which we would 
 refer the reader to histological treatises. We have already 
 had occasion to refer incidentally to many of the earliest 
 histological events ; and shall content ourselves by giving a 
 brief summary of the derivation of the tissues of the adult 
 animal from the three primary layers of the blastoderm. 
 
 The epiblast or upper layer of many embryologists forms 
 primarily two very important parts of the body, viz. the 
 central nervous system and the epidermis. 
 
 It is from the involuted epiblast of the neural tube, that 
 the whole of the grey matter of the brain and spinal cord 
 appears to be developed, the simple columnar cells of the 
 epiblast being apparently directly transformed into the 
 characteristic caudate nerve-cells. There is, however, some 
 doubt whether mesoblast cells may not possibly enter into 
 its formation, and it is very probable that the white matter 
 of the brain and spinal cord is derived from the mesoblast 
 alone. 
 
 The epithelium (ciliated in the young animal) lining the 
 canalis centralis of the spinal cord, together with that lining 
 the ventricles of the brain, all which cavities and canals are, 
 as we have seen, derivatives of the primary neural canal, is 
 the undifferentiated remnant of the primitive epiblast. 
 
 The epiblast as we have said also forms the epidermis ; 
 not however the dermis, which is of mesoblastic origin. 
 The line of junction between the epiblast and the meso- 
 blast coincides with that between the epidermis and the 
 dermis. From the epiblast are formed all such tegumentary 
 organs or parts of organs as are epidermic in nature. 
 
 In addition to these, the epiblast plays an important 
 part in the formation of the organs of special sense. 
 
 According to their mode of formation, these organs 
 may be arranged into two divisions. In the first come the 
 eases where the sensory expansion of the organ of special 
 
198 THE FIFTH DAY. [CHAP. 
 
 sense is derived from the involuted epiblast of the medullary 
 canal. To this class belongs the Retina, including the epi- 
 thelial pigment of the choroid, which is formed from the original 
 optic vesicle budded out from the fore-brain. 
 
 To the second class belong the epithelial expansions of 
 the membranous labyrinth of the ear, and the cavity of the 
 nose, which are formed by involution from the superficial 
 epiblast covering the external surface of the embryo. These 
 accordingly have no primary connection with the brain. We 
 may also fairly suppose that the 'taste bulbs' and the nervous 
 cells which have lately been described as present in the 
 epidermis are also structures formed from the epiblast. 
 
 In addition to these we have the crystalline lens formed of 
 involuted epiblast and the cavity of the mouth lined by it. 
 These are the most important parts which are derived from 
 the epiblast. 
 
 From the hypoblast are derived the epithelium of the 
 digestive canal, the epithelium of the trachea, bronchial 
 tubes and air cells, the cylindrical epithelium of the ducts 
 of the liver, pancreas and other glands of the alimentary canal, 
 as well as the hepatic cells constituting the parenchyma of 
 the liver, developed as we have seen from the solid hypoblast 
 cylinders given off around the primary hepatic diverticula. 
 
 Homologous probably with the hepatic cells, and equally 
 of hypoblastic origin, are the spheroidal i secreting cells' of 
 pancreas and other glands. The epithelium of the salivary 
 glands, though these so exactly resemble the pancreas, 
 is of epiblastic origin, inasmuch as the cavity of the mouth 
 (Chap. VI. 8) is entirely lined by epiblast. 
 
 The hypoblast also lines the allantois. 
 
 From the mesoblast are formed all the remaining parts 
 of the body. The muscles, the bones, the connective tissue 
 and the vessels, both arteries, veins, capillaries and lymphatics 
 with their appropriate epithelium, are entirely formed from 
 the mesoblast. 
 
 All the nerves of the body, both the cranial nerves (the 
 so-called optic and olfactory nerves alone excepted), the 
 spinal nerves, "and the sympathetic system, are also formed 
 from the mesoblast. The nerve-cells of the sympathetic 
 ganglia as well as those of the ganglia on the posterior roots 
 of the spinal nerves are of mesoblastic origin, and thus appar- 
 
VII.] THE MESOBLAST. 199 
 
 ently are in striking contrast with the nerve-cells in the brain 
 and cord. The fibres constituting the white matter of both 
 brain and spinal cord are also probably derived from mesoblast. 
 
 The generative and urinary organs are entirely derived 
 from the mesoblast. It is worthy of notice that their epithe- 
 lium, though resembling so closely the hypoblastic epithelium 
 of the alimentary canal, is distinctly mesoblastic. 
 
 From the mesoblast lastly are derived all the muscular, 
 conr^ective, and nervous and vascular elements, as well of the 
 alimentary canal and its appendages as of the skin and the 
 tegumentary organs. Just as it is only the epidermic moiety of 
 the latter which is derived from the epiblast, so it is only the 
 epithelium of the former which comes from the hypoblast. 
 
 In the present state of our knowledge we cannot in all 
 cases with certainty say which parts of the mesoblast enter 
 into the formation of particular organs ; the more important 
 facts in this part of our subject will however already have 
 been gathered, from the earlier part of this work. 
 
 11. The important events then which characterize the 
 fifth day are: 
 
 1. The growth of the allantois. 
 
 2. The appearance of the knee and elbow, and of the 
 cartilages which precede the bones of the digits and limbs. 
 
 3. The formation of the primitive cartilaginous cranium, 
 more especially of the investing mass, the trabeculse, and the 
 ethmo-vomerine plate ; and the appearance of rods of cartilage 
 in the visceral arches. 
 
 4. The developments of the parts cf the face: the 
 closing in of the nasal passages by the nasal processes. 
 
 5. The formation of the anus. 
 
 6. A large development of grey matter in the spinal 
 cord as the anterior and posterior cornua ; considerable growth 
 both of the anterior and posterior white columns, and the 
 commencement of the anterior and posterior fissures. 
 
 7. The appearance of the auricular septum, of a septum 
 in the bulbus arteriosus, and of the semilunar valves. 
 
 8. The establishment of the several tissues. 
 
CHAPTER VIII. 
 
 FROM THE SIXTH DAY TO THE END OF INCUBATION. 
 
 1. THE sixth day marks a new epoch in the development 
 of the chick, for distinctly avian characters then first make 
 their appearance. ' 
 
 Striking and numerous as are the features, which render 
 the class aves one of the most easily recognizable in the whole 
 animal kingdom, the embryo of a bird does not materially 
 differ in its early phases from that of a reptile or a mammal, 
 even in the points of structure which are most distinctively 
 avian. It may, it is true, be possible to infer, even at a com- 
 paratively early stage, from some subsidiary tokens, whether 
 any given embryo belongs to this class or that (and indeed the 
 same inference may be drawn from the ovum itself) ; but up to 
 a certain date it is impossible to point out, in the embryo of 
 the fowl, the presence of features which may be taken as 
 broadly characteristic of an avian organization. This absence 
 of any distinctive avian differentiation lasts in the chick 
 roughly speaking till the commencement of the sixth day. 
 
 We do not mean that on the sixth day all the organs 
 suddenly commence to exhibit peculiarities which mark 
 them as avian. There are no strongly marked breaks in the 
 history of development; its course is. perfectly gradual, and 
 one stage passes continuously into the next. The sixth and 
 seventh days do however mark the commencement of the 
 period in which the specialization of the bird begins to be 
 apparent. Then for the first time there become visible 
 the main features of the characteristic manus and pes ; the 
 crop and the intestinal caeca make their appearance; the 
 stomach takes on the form of a gizzard; the nose begins 
 
CHAP. VIII.] THE FCETAL APPENDAGES. 201 
 
 to develope into a beak ; and the commencing bones of the 
 skull arrange themselves after an avian type. Into these 
 details we do not propose to enter, and shall therefore 
 treat the history of the remaining days with great brevity. 
 
 We will first speak of the FCETAL APPENDAGES. 
 
 2. On the sixth and seventh days, these exhibit changes 
 which are hardly less important than the events of previous 
 days. 
 
 The amnion at its complete closure on the fourth day 
 very closely invested the body of the chick ; the true cavity of 
 the amnion was then therefore very small. On the fifth day, 
 fluid begins to collect in the cavity, and raises the membrane 
 of the amnion to some distance from the embryo. The 
 cavity becomes still larger by the sixth day, and on the 
 seventh day is of very considerable dimensions, the fluid 
 increasing with it. On the sixth day Von Baer observed 
 movements of the embryo, chiefly of the limbs ; he attributes 
 them to the stimulation of the cold air on opening the 
 egg. By the seventh day very obvious movements begin 
 to appear in the amnion itself; slow vermicular contractions 
 creep rythmically over it. The amnion in fact begins to 
 pulsate slowly and rythmically, and by its pulsation the 
 embryo is rocked to and fro in the egg. This pulsation is 
 due probably to the contraction of involuntary muscular 
 fibres, which seem to be present in the attenuated portion 
 of the mesoblast, forming part of the amniotic fold. (Cf. 
 Chap. n. 9, p. 42.) Similar movements are also seen in 
 the allantois at a considerably later period. 
 
 The growth of the allantois has been very rapid, and it 
 forms a flattened bag, covering the right side of the embryo 
 and rapidly spreading out in all directions, between the 
 primitive folds of the amnion, that is between the amnion 
 proper and the false amnion (or chorion). It is filled with 
 fluid, so that in spite of its flattened form its opposite walls 
 are distinctly separated from each other. 
 
 The vascular area has become still further extended than 
 on the previous day, but with a corresponding loss in the 
 definite character of its blood-vessels. The sinus terminalis 
 has indeed by the end of the seventh day lost all its 
 previous distinctness. And the vessels which brought back 
 the blood from it to the heart are no longer to be seen, 
 
202 THE SIXTH DAY. [CHAP. 
 
 Both the omphalo-mesaraie arteries and veins now pass 
 to and from the body of the chick as single trunks, assuming 
 more and more the appearance of being merely branches of 
 the mesenteric vessels. 
 
 The yolk is still more fluid than on the previous day, 
 and its bulk has (according to Von Baer) increased. This 
 can only be due to its absorbing the white of the egg, which 
 indeed is diminishing rapidly. 
 
 3. During the eighth, ninth, and tenth days, the 
 amnion does not undergo . any very important changes. 
 Its cavity is still filled with fluid, and on the eighth day 
 its pulsations are at their height, henceforward diminishing 
 in intensity. 
 
 The splitting of the mesoblast has now extended to the 
 outer limit of the vascular area, viz. over about three quarters 
 of the yolk-sac. The somatopleure at this point is continuous 
 (as can be easily seen by reference to Fig. 8) with the 
 original outer fold of the amnion. 
 
 It thus comes about that the further splitting of the 
 mesoblast merely enlarges the cavity in which the allantois 
 lies. The growth of this organ keeps pace with that of the 
 cavity in which it is placed. Spread out over the greater 
 part of the yolk-sac as a flattened bag filled with fluid, it 
 now serves as the chief organ of respiration. 
 
 Hence it is very vascular, the vessels on that side of the 
 bag which is turned to the chorion and shell being especially 
 large and numerous. 
 
 The yolk now begins to diminish rapidly in bulk. The 
 yolk-sac becomes flaccid, and on the eleventh day is thrown 
 into a series of internal folds, abundantly supplied with 
 blood-vessels. By this means the surface of absorption is 
 largely increased, and the yolk is more and more rapidly 
 taken up by the blood-vessels, and in a partially assimilated 
 condition transferred to the body of the embryo. 
 
 4. By the eleventh day the abdominal parietes though 
 still much looser and less firm than the walls of the chest 
 may be said to be definitely established, and the loops of 
 intestine, which have hitherto been hanging down into the, 
 somatic stalk, are henceforward confined within the cavity 
 of the abdomen. The body of the embryo is therefore 
 completed ; but it still remains connected with its various 
 
VTII.] THE ALLANTOIS. 203 
 
 appendages by a narrow somatic umbilicus, in which run 
 the stalk of the allantois and the solid cord suspending 
 the yolk-sac. 
 
 The cleavage of the rnesoblast still progressing, the yolk 
 is completely invested by the (splanchnopleuric) yolk-sac 
 except at the pole opposite to the embryo, where for some 
 little time a small portion remains unenclosed ; at this spot 
 the diminished white of the egg adheres as a dense viscid 
 plug, 
 
 The allantois meanwhile spreads out rapidly, and lies 
 over the embryo close under the shell, being separated from 
 the shell membrane by nothing more than an attenuated 
 membrane the chorion, formed out of the outer primitive 
 fold of the amnion and the remains of the vitelline mem- 
 brane. With this chorion the allantois partially coalesces, 
 and in opening an egg at the later stages of incubation, 
 unless care be taken the allantois is in danger of being torn 
 in the removal of the shell membrane. As the allantois in- 
 creases in size and importance, the umbilical (or allantoic) 
 vessels are correspondingly developed. They are very con- 
 spicuous when the egg is opened, the pulsations of the 
 umbilical arteries at once attracting attention. 
 
 5. On about the sixteenth day, the white having en- 
 tirely disappeared, the cleavage of the mesoblast is carried 
 right over the pole of the yolk opposite the embryo, and 
 is thus completed (Fig. 8). The yolk-sac now, like the 
 allantois which closely wraps it all round, lies loose in a 
 space bounded outside the body by the chorion, and con- 
 tinuous with the pleuroperitoneal cavity of the body of the 
 embryo. Deposits of urates now become abundant in the 
 allantoic fluid. 
 
 The loose and flaccid walls of the abdomen enclose a 
 space which the empty intestines are far from filling, and on 
 the nineteenth day the yolk-sac, diminished greatly in bulk 
 but still of some considerable size, is withdrawn through the 
 somatic stalk into the abdominal cavity, which it largely 
 distends. Outside the embryo there remains nothing now 
 but the highly vascular allantois and the practically blood- 
 less chorion and amnion. The amnion, whose fluid during 
 the later days of incubation rapidly diminishes, is continuous 
 at the umbilicus with the body-walls of the embryo. The 
 
204 THE SIXTH BAY. [CHAP. 
 
 chorion, (or outer primitive amniotic fold,) is by the comple- 
 tion of the cleavage of the mesoblast and the invagination 
 of the yolk-sac, entirely separated from the embryo. The 
 cavity of the allantois by means of its stalk passing through 
 the umbilicus is of course continuous with the cloaca. 
 
 6. In the EMBRYO itself a few general points deserve 
 notice. 
 
 By the sixth or seventh day, the flexure of the body has 
 become less marked, so that the head does not lie so near 
 to the tail as on the previous days; at the same time a 
 more distinct neck makes its appearance. 
 
 Though the head is still disproportionately large, its 
 growth ceases to be greater than that of the body. 
 
 Up to this period the walls of the somatic stalk have 
 remained thin and flaccid, almost membranous in fact, the 
 heart appearing to hang loosely out of the body of the 
 embryo. About this time however the stalk, especially in 
 front, rapidly narrows and its mesoblast becomes thickened. 
 In this way the heart and the other thoracic viscera are en- 
 closed by definite firm chest walls, along the sides of which 
 the ribs grow forwards and in front of which the cartilaginous 
 rudiments of the sternum appear. 
 
 The abdominal walls are also being formed, but not to 
 the same extent, and the stalk of the allantois still passes 
 out from the peritoneal cavity between the somatic and the 
 splanchnic stalks. 
 
 In the brain one of the most marked features is the 
 growth of the cerebral hemispheres. The median division 
 between these has in front increased in depth, so that 
 the lateral ventricles are continued forwards as two divergent 
 horns, while backwards they are also continued as similar 
 divergent horns separated from one another by the vesicle of 
 the third ventricle. 
 
 We propose to treat more fully of the development of the brain in a later 
 part of this work, the importance of the mammalian brain rendering it un- 
 desirable to go too much into the details of the brain of the bird. 
 
 All the visceral clefts, with the exception of the first, are 
 closed by the seventh day: this one however still remains 
 open, communicating with the mouth by the Eustachian tube 
 and with the exterior by the aperture of the external auditory 
 
VIII.] THE FEATHERS. 205 
 
 meatus. It becomes divided internally into two parts by the 
 tympanic membrane. 
 
 The structures which surround the mouth are beginning 
 to become avian in form, though the features are as yet not 
 very distinctly marked. The inferior maxillary processes 
 meet in front and form the lower boundary of the mouth ; 
 while, separated from these by only a narrow slit, the superior 
 maxillary processes and fronto-nasal process meet in a similar 
 way above, to form the upper boundary. The union of the 
 superior maxillary processes is not with the tip of the fronto- 
 nasal process, but with its sides, so that an angular space is 
 left on each side between them (vide Fig. 58). The nasal 
 grooves are however completely roofed over. 
 
 The tongue has appeared on the floor of the mouth as a 
 bud of mesoblast covered by epiblast. 
 
 7. During the eighth, ninth, and tenth days, the 
 embryo grows very rapidly, the head being still especially 
 large, and at the same time becoming more round, the mid- 
 brain not being so prominent. 
 
 8. From the eleventh day onwards the embryo suc- 
 cessively puts on characters which are not only avian,- but 
 even distinctive of the genus, species and variety. 
 
 So early as the ninth or tenth day the sacs containing 
 the feathers begin to protrude from the surface of the skin as 
 papillae especially prominent at first along the middle line of 
 the back from the neck to the rump, and over the thighs, 
 the sacs of the tail feathers being very conspicuous. On the 
 thirteenth day, these sacs, generally distributed over the 
 body, and acquiring the length of a quarter of an inch or 
 more, appear to the naked eye as feathers, the thin walls of 
 the sacs allowing their contents, now coloured according to 
 the variety of the bird, to shine through. They are still 
 however closed sacs, and indeed remain such even on the 
 nineteenth day when many of them are an inch in length. 
 
 On the eighth day a chalky-looking patch is observable 
 on the tip of the nose. This by the twelfth day has become 
 developed into a horny but still soft beak. 
 
 On the thirteenth day, nails are visible at the ex- 
 tremities, and scales on the remaining portions of the toes. 
 These on the sixteenth day become harder and more horny, 
 as does also the beak. 
 
206 
 
 THE SIXTH DAY. 
 
 [CHAP. 
 
 By the thirteenth day the cartilaginous skeleton is com- 
 pleted and the various muscles of the body can be made out 
 with tolerable clearness. 
 
 Ossification begins according to Von Baer on the eighth 
 or ninth day by small deposits in the tibia, in the metacarpal 
 bones of the hind- limb, and in the scapula. On the eleventh or 
 twelfth day a multitude of points of ossification make their 
 appearance in the limbs, in the scapular and pelvic arches, in 
 the ribs, in the bodies of the cervical and dorsal vertebraB and 
 in the bones of the head, the centres of ossification of the 
 vertebral arches not being found till the thirteenth day. 
 
 FIG. 62. 
 
 - DIAGRAM OF THE VENOUS CIRCULATION AT THE COMMENCEMENT OF THE 
 
 FIFTH DAY. 
 
 JI. heart. D. C. ductus Cuvieri. Into the ductus Cuvieri of each side fall /. the 
 jugular vein, Su. V. the superior vertebral, W. the vein from the wing and 
 C. the inferior cardinal vain. S. V. sinus venosus. Of. omphalo-mesaraic 
 vein. U. umbilical vein, which at this stage gives off branches to the 
 body- walls. V. 0. 1. inferior vena cava. 
 
VIII.] THE VENA CAVA INFERIOR. 207 
 
 9. The events which we have thus briefly narrated are 
 accompanied by important changes in the arterial and 
 venous systems. 
 
 The condition of the venous system at about the end 
 of the third day was fully described in Chap. V. 16, and 
 the changes which have taken place between that date and 
 the latter days of incubation may be seen by comparing 
 the diagram Fig. 39 B with the diagrams Figs. 62 and 63. 
 
 On the third day, nearly the whole of the venous blood 
 from the body of the embryo was carried back to the heart by 
 two main venous trunks, the superior (Fig. 39 B, Su. V.) and 
 inferior (Fig. 39 B, (7) cardinal veins, joining on each side 
 to form the short transverse ductus Cuvieri, both which in 
 turn united with the sinus venosus close to the heart. As 
 the head and neck continue to enlarge and the wings become 
 developed, the single superior cardinal or jugular vein, 
 as it is usually called (Fig. 62, J"), of each side, is joined 
 by two new veins : the vertebral vein (Su. V.), bringing 
 back blood from the head and neck, and the vein from the 
 wing (W). 
 
 The inferior cardinal or vertebral veins have their roots 
 in the Wolffian bodies ; they become developed, pari passu, 
 with those organs, and may be called the veins of the 
 Wolffian bodies. On the third day they are the only veins 
 which bring the blood back from the hinder part of the body 
 of the embryo. 
 
 About the fourth or fifth day, however, a new single 
 venous trunk, the vena cava inferior (Fig. 62, V. C. /.), makes 
 its appearance in the middle line, in a plane more dorsal 
 than that of the cardinal veins. This, starting from the 
 sinus venosus not far from the heart, is on the fifth day a 
 short trunk running backward in the middle line below the 
 aorta, and speedily losing itself in the tissues above the 
 Wolffian bodies. When the kidneys are formed it receives 
 blood from them, and thenceforward enlarging rapidly be- 
 comes the channel by which the greater part of the blood 
 from the hind limbs and the hinder part of the body finds its 
 way to the heart. In proportion as this vena cava inferior 
 increases in size, and the Wolffian bodies give place to the 
 permanent kidneys, the posterior cardinal veins diminish. 
 Communicating branches between them and it are established 
 
208 
 
 THE SIXTH DAY. 
 FIG. 63. 
 
 [CHAP. 
 
 -JSVY. 
 
 DIAGRAM OF THE VENOUS CIRCULATION DURING THE LATER DAYS OP 
 INCUBATION. 
 
 II. heart. V. S. R. right vena cava superior. V. S. L. left vena cava superior. 
 S. V. sinus venosus. The two venae cavee superiores are the original 
 'ductus Cuvieri/ they still open into the sinus venosus and not in- 
 dependently into the heart. /. jugular vein. 817. V. superior vertebral 
 vein. W. vein for the wing. V. C.I. vena cava inferior, which receives 
 most of the blood from the inferior extremities, etc. HP. Hepatic veins, 
 which fall into the vena cava inferior. D. V. ductus venosus. P. V. portal 
 vein. M. a vein bringing blood from the intestines into the portal vein. 
 Of. omphalo-mesaraic vein. U. umbilical vein. The three last mentioned 
 veins unite together to form the portal vein. 
 The remnants of the inferior cardinal veins are not shewn. 
 
VIII.] THE VEN.E CAV^E. 209 
 
 in the substance of the Wolffian bodies, so that soon the 
 blood of those organs, as well as that from all the rest of the 
 hind body, with the exception of the alimentary canal and its 
 appendages, passes into the vena cava inferior. 
 
 The diminished trunks of the cardinal veins remain for 
 some time ; their anterior ends unite to form the small azygos 
 vein. 
 
 At its first appearance the vena cava inferior may be con- 
 sidered as a branch of the trunk which we have called the 
 sinus venosus, but as development proceeds, and the vena 
 cava becomes larger and larger, the sinus venosus assumes 
 more and more the appearance of being merely the cardiac 
 termination of the vena cava, and the ductus venosus may 
 now be said to join the vena cava instead of being prolonged 
 into the sinus. 
 
 While this growth of the vena cava is going on, the points 
 at which the ductus Cuvieri enter into the sinus venosus, or, 
 as we may now say, vena cava inferior, are drawn in towards 
 the heart itself, and finally these trunks fall directly and 
 separately into the auricular cavities, and are henceforward 
 known as the right and left vena cava superior (Fig. 63, V.S.R., 
 V.S.L.}. There are therefore, when these changes have been 
 effected, three separate channels, with their respective orifices, 
 by which the blood of the body is brought back to the heart, 
 viz. the right and left superior and the inferior venae cavae. 
 
 While the auricular septum is as yet unformed, the blood 
 from these veins falls into both auricles, perhaps more into 
 the left than into the right. As the septum however grows 
 up, the three vessels become connected with the right auricle 
 only while the left receives the two pulmonary veins coming 
 from the lungs. (Compare Chap. vn. 7.) 
 
 On the third day the course of the vessels from the yolk- 
 sac is very simple. The two omphalo-mesaraic veins, of which 
 the right is already the smaller, form the meatus venosus 
 from which, as it passes through the liver on its way to the 
 heart, are given off the two sets of vence advehentes and venae 
 revehentes. 
 
 With the appearance of the allantois on the fourth day, 
 
 a new feature is introduced. From the .meatus venosus, 
 
 a short distance behind the liver, there is given off a vein 
 
 which quickly divides into two branches. These, running 
 
 E. 14 
 
210 THE SIXTH DAY. [CHAP. 
 
 along the ventral side of the body from the walls of 
 which they receive some amount of blood, pass to the 
 allantois. They are the allantoic or umbilical veins. The 
 single vein which they unite to form becomes, by reason 
 of the rapid growth of the allantois, very long ; and hence 
 it is perhaps better to speak of it as the umbilical vein 
 (Fig. 63, u). The right branch soon diminishes in size 
 and finally disappears. Meanwhile the left on reaching the 
 allantois bifurcates ; and, its two branches becoming large 
 and conspicuous, there still appear to be two main allantoic 
 or umbilical veins uniting at a short distance from the 
 allantois to form the single long umbilical vein. At its 
 first appearance the umbilical vein- seems to be but a small 
 branch of the omphalo-mesaraic, but as the allantois grows 
 rapidly, and the yolk-sac dwindles, this state of things is 
 reversed, and the less conspicuous omphalo-mesaraic appears 
 as a branch of the larger umbilical. 
 
 On the third day the blood returning from the walls 
 of the intestine is insignificant in amount. As however the 
 intestine becomes more and more developed, it acquires a 
 distinct venous system, and the blood sent to it by branches 
 of the aorta is returned by veins which form a trunk, the 
 mesenteric vein (Fig. 63, M), falling into the omphalo-mesaraic 
 vein at its junction with the umbilical vein. 
 
 These three great veins in fact, viz. the omphalo-mesaraic, 
 the umbilical, and the mesenteric, form a large common trunk 
 which enters at once into the liver, and which we may now 
 call the portal vein (Fig. 63, P. V.}. This, at its entrance into 
 the liver, partly breaks up into the vence advehentes, and partly 
 continues as the ductus venosus straight through the liver, 
 emerging from which it joins the vena cava inferior. Before 
 the establishment of the vena cava inferior, the venae reve- 
 hentes, carrying back the blood which circulates through the 
 hepatic capillaries, joined the ductus venosus close to its exit 
 from the liver. By the time however that the vena cava has 
 become a large and important vessel it is found that the 
 venae revehentes or as we may now call them the hepatic veins 
 have shifted their embouchment and now fall directly into 
 that vein, the ductus venosus making a separate junction 
 rather higher up (Fig. 63, HP). 
 
 This state of things continues with but slight changes till 
 
VIII.] 
 
 THE VENOUS CIKCULATION. 
 FIG. 64. 
 
 211 
 
 DIAGRAM OF THE VENOUS CIRCULATION OP THE CHICK AFTER THE COMMENCE- 
 MENT OF RESPIRATION BY MEANS OF THE LUNGS. 
 
 W. wing- vein, /.jugular vein. Su. V. superior vertebral vein. These unite 
 together on each side to form the corresponding superior vena cava. 
 L. V. pulmonary veins. V. C. I. vena cava inferior. HP. hepatic veins. 
 P. V. portal vein. M. mesenteric veins. At U. and O.f. are shewn the 
 points at which the umbilical and omphalo-mesaraic veins originated 
 previous to their becoming obliterated at the commencement of respiration. 
 Co. V. connecting vessel between the branches of the portal vein and the 
 vena cava inferior. It is called the coccygeo-mesenteric vein, and unites 
 the cross branch connecting the two hypogastrics with the mesenteric 
 vein. It is represented in the figure in a purely diagrammatic manner. 
 The ductus venosus has become obliterated. The three venae cavse fall 
 independently into the right auricle and the pulmonary veins into the 
 left auricle. 
 
 142 
 
212 
 
 THE SIXTH BAY. 
 
 [CHAP. 
 
 near the end of incubation, when the chick begins to breathe 
 the air in the air-chamber of the shell, and respiration is no 
 longer carried on by the allantois. Blood then ceases to flow 
 along the umbilical vessels ; they become obliterated. The 
 omphalo-mesaraic vein, which as the yolk becomes gradually 
 absorbed proportionately diminishes in size and importance, 
 comes to appear as a mere branch of the portal vein. The 
 ductus venosus becomes closed, remaining often as a mere 
 ligament ; and hence the whole of the blood coming through 
 the portal vein flows into the substance of the liver, and so 
 by the two hepatic veins into the vena cava (Fig. 64, HP). 
 
 Previous to these changes one of the veins passing from 
 the rectum into the vena cava has given off a branch which 
 effects a junction with one of the mesenteric veins. This now 
 forms a somewhat conspicuous connecting branch between the 
 systems of the vena cava and the portal vein (Fig. 64, Co. V.). 
 
 All three venae eava3 now fall exclusively into the right 
 auricle, and by the closure of the foramen ovale the blood 
 
 J.CA. 
 
 I.CA 
 
 VA- 
 
 STATB OF ARTERIAL CIRCULATION ON THE FIFTH OR SIXTH BAT, 
 
 E. CA. external carotid. /. CA. internal carotid. AO. dorsal aorta, icf. A* 
 arteries to the Wolffian bodies. Ver. A. arteries given off between each 
 pair of vertebrae. Of. A. omphalo-mesaraic artery. UA. umbilical artery. 
 I A . iliac artery. 
 
VIIL] THE CAKOTIDS. 213 
 
 flowing through them is entirely shut off from the left auricle, 
 into which passes the blood from the two pulmonary veins 
 (Fig. 64,i,F.). 
 
 Such is the history of the veins in the chick. As will be 
 seen in the second part of this work, the course of events in 
 the mammal, though in the main similar, differs in some 
 unimportant respects. 
 
 10. It remains for us to speak of the changes which have 
 in the meantime been taking place in the arterial system. The 
 condition of things which exists on the fifth or sixth day is 
 shewn in the diagram (Fig. 65). 
 
 We have already seen (Chap. VI. p. 168) that of the three 
 aortic arches which make their appearance on the third day, 
 the first two disappear : the first on the fourth, the second on 
 the fifth day ; but that their disappearance is accompanied 
 by the formation behind them of two new aortic arches, the 
 fourth and the fifth. Thus there are generally three, never 
 more than three, pairs of aortic arches present and functional 
 at one time. 
 
 According however to Yon Baer this is not strictly true. He states that 
 there are four arches present both on the fourth and fifth days. In the case 
 of the fourth day, a slight remnant of the first pair of arches still persists when 
 the fourth pair is already formed ; and on the fifth day the third pair has not 
 entirely disappeared when the fifth pair is formed. In both of these cases 
 however the first pair of arches of the four is only present for a very short time, 
 and then is so diminished in size as to be of no importance. 
 
 The first pair of arches, before it entirely disappears, sends 
 off on each side two branches towards the head. Of these, 
 one forms the direct continuation of the bulbus arteriosus in 
 a straight line from the point where the first aortic arch 
 leaves it. Primarily distributed to the tongue and inferior 
 maxillary region, it becomes the external carotid (Fig. 65, 
 
 The other, starting from the point where the aortic arch 
 of each side joins its fellow above the alimentary canal to form 
 the dorsal aorta, is primarily distributed to the brain, and 
 becomes the internal carotid (Fig. 65, 1.CA.). 
 
 When the first arch disappears, the external carotid 
 arteries still remain as the anterior continuations of the 
 bulbus arteriosus. And since the dorsal trunks uniting the 
 distal ends of the first and second arches do not become 
 obliterated at the time when the first pair of arches dis- 
 
214 THE SIXTH DAY. [CHAP. 
 
 appears, the internal carotids remain as branches springing 
 from the distal ends of the second pair of arches ; they are 
 supplied with blood from that pair, the stream in which flows 
 chiefly towards the head instead of backwards towards the 
 dorsal aorta, as is the case with the succeeding arches. When 
 the second pair of arches is obliterated, the connecting branch 
 with the next arch is again left, and thus the internal carotids 
 appear as branches' from the distal ends of the third pair of 
 arches. 
 
 On the third day, the dorsal aorta does not long remain 
 single in its backward course along the body, but soon 
 divides into two trunks which run one on either side of the 
 middle line of the body. On the fourth day however, the 
 point at which the aorta divides is carried very much further 
 back, .quite to the posterior end of the Wolffian bodies. The 
 two branches into which it there divides, form the origin 
 of the iliac (Fig. 65, 1 A) arteries supplying the hind limbs. 
 Each of these sends a short branch to the allantois (UA}. 
 As the allantois grows rapidly and becomes an import- 
 ant respiratory organ, these allantoic or umbilical arteries 
 increase so much in size that they speedily appear to be the 
 direct continuations of the aorta, and the iliac arteries to be 
 mere branches of them. As a general, though apparently 
 not invariable rule, the right umbilical artery gets gradually 
 smaller and soon disappears. 
 
 From the main trunk of the aorta are given off small 
 transverse branches between the vertebrae (represented dia- 
 grammaticallyin Fig. 65, Ver. A.}, and also important branches 
 to the Wolffian bodies (Fig. 65, wf. A.}. 
 
 The omphalo-mesaraic artery ( Of. A.) now leaves the aorta 
 as a single but quickly bifurcating trunk, which at the end 
 of the fifth day is still very large. 
 
 By the fifth day, the ventricular portion of the heart 
 (compare Chap. vn. 7) is almost completely divided into 
 two chambers. The bulbus arteriosus is also divided by a 
 septum into two channels, which do not however run in s a 
 straight course, but have, according to Von Baer, a spiral 
 arrangement. One of the channels communicates with the 
 right ventricle of the heart and the other with the left. The 
 spiral of the former turns from the right and above (dorsal), 
 to the left and below (ventral) ; the latter from the left and 
 
VIII.] THE SEPTUM OF THE BULBUS ARTEKIOSUS. 215 
 
 below (ventral), to the right and above (dorsal). The septum 
 separating these two channels, according to Dr Tonge (Proc. 
 Roy. Soc. 1868), commences as an outgrowth from the wall 
 between the fourth and fifth pair of arches, and is so arranged 
 that the channel from the left ventricle communicates with 
 the third and fourth pairs of arches only, and that from the 
 right ventricle with the fifth pair only. 
 
 According to Dr Tonge's view the two channels after the completion of the 
 septum do not communicate with each other at any point, and are completely 
 shut off from each other at their ends. Von Baer believed that the two 
 channels (into which the bulbus arteriosus is divided by the septum) opened at 
 their distal terminations into a common trunk, but that the direction of the 
 openings of the two channels caused the two streams of blood from them to 
 enter into different arterial arches. According to him the stream from the 
 channel communicating with the left ventricle is directed so as entirely to 
 miss the last pair of arterial arches, and to fall into the third and fourth pairs, 
 while that from the right ventricle enters the fifth pair alone. 
 
 One result of this arrangement is that all the blood which 
 passes to the anterior extremity of the body, comes from the 
 left ventricle of the heart. 
 
 At about the seventh day an entire separation begins to 
 take place between the arterial roots which come respectively 
 from the right and left chambers of the heart. The root 
 from the right chamber (Fig. 66, R.P.A.) remains connected 
 with the fifth pair of arches. The root from the left ventricle 
 is connected with the third and fourth pairs of arches. 
 
 According to Von Baer the right arterial root is connected with the fourth 
 arch on the left side, and the fifth arch on the right side. He also believed 
 that the fifth arch on the left side has by this day altogether disappeared. 
 According to his view, therefore, the pulmonary arteries are derived from 
 both the fifth and fourth pairs of arches. Rathke (Denkschriften der AJcademie 
 zu Wien, 1857, Bd. xin.) however altogether denies this, and states that both 
 pulmonary arteries are derived from the fifth pair of arches alone. We have 
 in our account followed Rathke's statements. 
 
 The lower part of the body still receives blood from both 
 the right and left ventricles, since the blood which enters 
 the fifth arch still flows into the common dorsal aorta. 
 
 As the lungs however increase in size, a communication 
 is set up between them and the fifth pair of arches in the 
 shape of two vessels which, springing one from the arch of 
 each side, grow downwards towards the lungs. At first 
 small and narrow, these pulmonary arteries, for such they 
 are, grow rapidly larger and larger, so that more and more 
 of the blood from the right ventricle is carried to the lungs. 
 
216 
 
 THE SIXTH DAY. 
 
 [CHAP. 
 
 FIG. 66. 
 
 JS.CA 
 
 DIAGRAM OF THE CONDITION OF THE ARCHES OF THE AORTA TOWARDS THE 
 CLOSE OF INCUBATION. 
 
 J* 2 ) 3>4> 5- The several aortic arches. E.CA. External carotid. I.Ca. Internal 
 carotid. O. C.A. Common carotid. V.A. 'Vertebral' artery. J2.sc. Eight 
 subclavian. L.sc. Left subclavian. L.I.N. Left innominate. AO. Aorta. 
 P. A. Pulmonary arteries. R.P.A. Right arterial root, or division of 
 bulbus arteriosus, or pulmonary artery ; the left root or division, con- 
 stituting the aorta, is seen by its side. RC.b. Communication on the 
 right side between the fourth and fifth arches. L.C.b. Communicating 
 trunk between the fifth arch and the dorsal aorta. 
 
 At the same time the connection between the third and 
 fourth pairs of arches on each side grows weaker ; and finally 
 the passage between them becomes obliterated, so that less 
 and less of the blood which flows along the third pair of 
 arches is able to pass backwards to the hind end of the 
 body. 
 
 On the eighth day (according to Rathke, loc. cit.) from the common root of the 
 external and internal carotids of each side, a branch (Fig. 66, V.A.) is given 
 off" which passes forwards along the neck, and ends in front by becoming 
 connected with a branch from the external carotid. This vessel is of 
 much larger calibre at its two extremities, than in the central part of its 
 course. In the adult it terminates anteriorly by anastomosing with the 
 occipital artery (a branch from the external carotid). The smaller calibre of 
 the central part is still more marked in the adult, than at the stage we are 
 
VIII.] THE SUBCLAVIAN ARTERIES. 217 
 
 describing. This vessel "passes up in the neural arch of the vertebrae, and is 
 usually spoken of as the vertebral artery. 
 
 Rathke calls the vertebral arteries the 'arterise collaterals colli,' and if his 
 view of their development is correct they can scarcely be considered homologous 
 with the vertebral arteries usually so called in mammals. Soon after the ana- 
 stomosis between the third and fourth arches disappears, the common trunk of 
 the carotids on each side becomes much lengthened, and it is from near the base 
 of the lengthened common carotid that the 'vertebral artery' (as we shall call 
 Rathke's ' arteria collateralis colli ') takes its origin. 
 
 The fourth arch of the right side now becomes the most 
 important of all the arches ; and nearly the whole of the 
 blood supplying the hinder parts of the body passes through 
 it. It is this arch which remains as the permanent aortic 
 arch of the adult; and it is important to notice that the 
 arch which forms the great dorsal aorta in birds is the fourth 
 on the right side, and not as in mammals the fourth on the 
 left side. 
 
 From the observations of Rathke we know, with tolerable certainty, the 
 manner in which the carotids and the so-called vertebral arteries of birds are 
 developed. There is however still some doubt as to the origin of the subclavian 
 arteries, although Von Baer and Rathke have both investigated the point. 
 
 Von Baer believed that the artery which forms the continuation of the third 
 arch, which on Rathke's authority we have called the * internal carotid,' 
 became the 'vertebral artery;' and he believed that the subclavian was given 
 off as a branch from it. As Rathke points out, this does not agree with the 
 anatomy of the parts, since in birds the subclavian forms the continuation of 
 the innominate artery, after a common branch for the vertebral and carotids 
 has been given off from it. If it had not been for Rathke's satisfactory 
 observations on the development of the carotids and vertebrals, this would not 
 be a fatal objection to Von Baer's view; since we might easily suppose that 
 although the subclavian was originally a branch of the vertebral, yet by sub- 
 sequent changes, the point at which the subclavian left the vertebral was 
 carried further and further back, till finally the subclavian became a branch 
 of the common trunk of the vertebral and carotids, or in other words the 
 subclavian formed the continuation of the innominate artery, after the com- 
 mon branch which divides into the vertebral and carotids had been given off 
 from it. 
 
 Rathke's view of the origin of the subclavian is founded on the analogy of 
 other vertebrates, rather than on his own observations on the chick. He states 
 that although he attempted to do so, he was unable satisfactorily to observe the 
 origin of these vessels in the chick. The following is the view which he adopts, 
 and which we have followed in our diagram (Fig. 66). 
 
 The right subclavian (R.sc.) arises, he believes, either from the connecting 
 branch (anastomosis) between the fourth and fifth arches (of that side), or from 
 the branch connecting the fifth arch with the dorsal aorta ; probably from the 
 former. This would make its development very nearly similar to the develop- 
 ment of the corresponding subclavian (i.e. the left) in mammals. We shall 
 mention directly, in speaking of the final changes which the arterial system 
 undergoes, how it is that the right subclavian finally comes to form the continu- 
 ation of the right innominate artery. 
 
 Tha left subclavian (L. sc.) forms the continuation of the fourth arch 
 
218 THE SIXTH DAY. [CHAP. 
 
 of the left side, and primarily takes its origin from the connecting branch 
 between the fourth and fifth arches. Its mode of development thus nearly 
 agrees with that of the right subclavian of mammals. In support of this view 
 of the development of the left subclavian, is the fact that in some birds there 
 is, between it and the dorsal aorta, a fibrous connection which is the remains of 
 the vessel which originally carried the blood from this fourth arch to the dorsal 
 aorta. On the left side therefore, as can easily be understood by reference to 
 the diagram, the subclaviau without further alteration remains as the continu- 
 ation of the left innominate, L.I.N. 
 
 In consequence of these changes the condition of the 
 aortic arches during the latter days of incubation, before 
 respiration by the lungs has commenced, is as follows 
 (Fig. 66). 
 
 The first and second arches are completely obliterated. 
 The third arch on each side is continued at its distal end as 
 the internal carotid, J.(7a, the connection between it and 
 the fourth arch having become entirely obliterated. From 
 its proximal end as the direct continuation of the trunk which 
 originally supplied the first and second arches the external 
 carotid, E.CA., is given off. Each pair of carotids arises 
 therefore from a common trunk the common carotid (C. 
 G.A.}. Each of these trunks gives off near its proximal 
 end a branch, the vertebral artery (V.A.), which joins at 
 its distal end a branch from the external carotid. 
 
 The common carotid on the right side comes off from the 
 fourth arch of the right side (the arch of the dorsal aorta), 
 and is not as yet connected with the right subclavian, R.sc. 
 
 The common carotid of the left side comes off from the 
 fourth arch of the left side ; but since this arch becomes the 
 left subclavian, L.sc. (the connection between the fourth and 
 fifth left arches being obliterated), the portion of the trunk 
 (L.I.N.) between the fourth arch and the bulbus arteriosus (or 
 as it must now be called the common aortic root) is called 
 the left innominate artery. 
 
 The fourth arch of the right side forms the commence- 
 ment of the great dorsal aorta, and gives off the right 
 subclavian (R. sc.) just before it is joined by the fifth arch. 
 
 The fifth arch of each side gives off branches P.A. to the 
 lungs ; their distal continuations RC.b., L. C.b. y by which they 
 are connected with the systemic circulation, though much 
 reduced, are not obliterated. 
 
 The final changes undergone by the arterial system 
 after the commencement of the pulmonary respiration consist 
 
VIII.] 
 
 THE DUCTUS BOTALLI. 
 
 219 
 
 chiefly in the complete separation of the pulmonary and 
 systemic circulations. 
 
 RIN 
 
 AO 
 
 DlAGKAM OF THE ABTEKIAL SYSTEM OF THE ADULT FOWL. 
 
 R.I.N. right innominate artery. The other letters as in Fig. 66. 
 The dotted lines, as before, shew the portions of arches which have been 
 obliterated. 
 
 As the branches to the lungs become stronger and 
 stronger, less and less blood from the right ventricle enters 
 into the dorsal aorta; and the connecting vessels become 
 smaller and smaller. 
 
 Each of the arches from the right ventricle may therefore 
 be considered at about the sixteenth or eighteenth day as 
 divided into two parts, an inner part which connects the heart 
 with the lung, and an outer part which still connects the arch ' 
 with the main dorsal aorta. As these outer parts become 
 smaller they receive the name of the 'ductus or canales 
 Botalli' or 'ductus arteriosi.' The one on the right side is 
 short ; that on the left side is much longer and narrower. 
 
 Von Baer supposed that the reason of this was, that since the pulmonary 
 arch of the left side was the fourth, the ductus Botalli of that side consisted of 
 the branch between the fourth and fifth arch, as well as between the fifth arch and 
 the dorsal aorta. It is easy however from diagram (Fig. 66) to see that this 
 
220 THE SIXTH DAY. [CHAP. 
 
 reason is superfluous, and that the explanation is that the canalis Botalli of the 
 right side is merely the portion of the fifth arch between the origin of the 
 pulmonary artery and the junction of the fifth arch with that common trunk, 
 with which all the arches of that side are at one time or other connected, 
 and which remains as the continuation of the aortic arch into the dorsal aorta; 
 while on the left side it consists of the same portion of the arch as on the right 
 side, and also of the corresponding common trunk of the left side. 
 
 When respiration commences the blood ceases to pass 
 through these canals, which either remain as mere ligaments 
 or else become absorbed altogether. By this means, the 
 foramen ovale becoming at the same time closed, a complete 
 double circulation is established. All the blood from the 
 right ventricle passes into the lungs, and all that from the 
 left ventricle into the body at large. 
 
 Two other changes take place about the same time in 
 the aortic branches. That portion of the right fourth or 
 aortic arch which lies between the origin of the right sub- 
 clavian and the common carotid becomes shortened, and 
 is finally swallowed up in such a fashion that the right 
 subclavian (Fig. 67, R.sc.) comes off from the right common 
 carotid, a very short trunk being left between the union of 
 the two to serve as the right innominate artery. 
 
 At the same time, corresponding to the increase in the 
 length of the neck, the common carotids are very greatly 
 lengthened. They lie close together in the neck, and in 
 many birds actually unite to form a common trunk. 
 
 It will of course be understood that with the disap- 
 pearance of the allantois and the absorption of the yolk, 
 the umbilical and omphalo-mesaraic arteries also disappear. 
 
 11. It may perhaps be of advantage to the reader if we 
 here briefly summarize the condition of the circulation at 
 its four most important epochs; viz. on the third day, on 
 the fifth day, during the later days of incubation before 
 respiration by the lungs has commenced, and after the chick 
 has begun to breathe by the lungs. 
 
 On the third day the circulation is of an exceedingly 
 simple character. 
 
 The heart is to all intents and purposes a simple twisted 
 tube marked off by constrictions into a series of three con- 
 secutive chambers. The blood coming from the venous 
 radicles passes through the heart and then through the 
 three pair of arterial arches. 
 
VIII.] THE CIRCULATION ON THE FIFTH DAY. 221 
 
 From these it is collected into the great dorsal aorta. 
 On this dividing into two branches, the stream of blood also 
 divides and passes down on each side of the notochord along 
 the body, and thence out by the omphalo-mesaraic arteries, 
 which distribute it to the yolk-sac. 
 
 In the yolk-sac it partly passes into the sinus terminalis 
 and so into the fore and aft trunks, partly directly into the 
 lateral trunks, of the omphalo-mesaraic veins. In both cases 
 it is brought back to the two venous radicles and so to the 
 heart. 
 
 On this day the blood is aerated in the capillaries of the 
 yolk-sac. 
 
 On the fifth or sixth day, the two auricles are present 
 though having a common cavity. The septum of the ven- 
 tricles is nearly complete, so that the blood on entering the 
 ventricles from the auricles is divided into two streams. 
 These two streams pass respectively from the right and left 
 chambers of the heart into the two divisions of the bulbus 
 arteriosus. The blood from the right ventricle passes into 
 the fifth pair of arches and that from the left ventricle into 
 the third and fourth pairs of arches. 
 
 From the anterior parts the blood is brought back by the 
 anterior cardinal or jugular veins ; from the hinder parts of 
 the body, chiefly by the cardinal veins, but also in part by 
 the now commencing vena cava inferior. 
 
 The blood from the yolk-sac and allantois, together with 
 a small quantity from the intestine, is collected into the 
 portal vein, and by that vessel carried to the liver. Here it 
 becomes divided into two streams, part flowing directly by 
 the ductus venosus into the sinus venosus, and the remainder 
 passing through the capillaries of the liver and being brought 
 back to the sinus venosus by the hepatic veins. 
 
 During this period the blood is aerated both by the 
 allantois and yolk-sac, but as yet chiefly by the latter. 
 
 At a somewhat late period of incubation, the blood from 
 the ventricles passes into two entirely distinct roots. The 
 one of these, that from the right chamber, sends the blood to 
 the fifth pair of arches. Passing through these two arches, 
 the greater part of the blood flows into the dorsal aorta, a 
 small portion only finding its way into the lungs through the 
 as yet unimportant pulmonary arteries. 
 
222 THE SIXTH DAY. [CHAP. 
 
 Through the other aortic root, viz. that from the left 
 ventricle, the blood flows into the third and fourth pairs of 
 arches. That part of the blood which flows into the third 
 pair of arches, passes almost entirely to the head and upper 
 extremities by the external and internal carotids ; that which 
 flows into the right arch of the fourth pair, is chiefly brought 
 to the dorsal aorta, but some of it passes to the right wing ; 
 that, on the contrary, which goes into the left fourth arch, is 
 for the most part sent to the wings, a small part only reaching 
 the dorsal aorta. There is still a mixture of the blood from 
 the two chambers of the heart, so that the blood in the 
 dorsal aorta is composed partly of the blood from the Ieft 5 
 and partly from the right chambers. The blood of the upper 
 (anterior) end of the body comes entirely from the left ventricle. 
 
 The blood of the dorsal aorta passes to the yolk-sac and 
 allantois, and to all the hinder parts of the body. It 
 is brought back from the yolk-sac, from the allantois, and 
 to a certain extent from the intestines, by the portal vein, 
 part of the blood from which passes to the inferior vena cava 
 by the direct course (ductus venosus), and part indirectly by 
 the more circuitous course of the capillaries of the liver and 
 hepatic veins. 
 
 The blood from the generative and urinary organs, and 
 from the hinder extremities, is brought back to the heart by 
 the vena cava inferior ; that from the upper extremities and 
 head by the jugular, vertebral and wing veins into the two 
 venae cavae of the right and left side, and so to the heart. 
 Of these three venae cavae, the left superior and the inferior 
 open into the heart independently. The right superior enters 
 with the inferior. All of these enter the right auricle, but 
 the common opening of the inferior and right superior venae 
 cavae is so directed, that the blood carried by those vessels 
 flows chiefly through the foramen ovale into the left auricle. 
 The blood from the left superior vena cava enters the right 
 auricle only. Now the blood of the inferior vena cava 
 has been partly aerated by the allantois ; and, since it is this 
 blood which passing through the left auricle and ventricle is 
 distributed to the third aortic arch, unmixed by any blood 
 from the right ventricle (the mixture with the blood from the 
 fifth arch taking place in the fourth arch only), it happens 
 that the blood which flows to the anterior extremities and 
 
VIII.] THE ADULT CIRCULATION. 223 
 
 head, is more aerated than that in any other part of the 
 body. 
 
 From the anterior extremities the blood is to a great 
 extent returned by the left superior cava, and goes into the 
 right auricle, whence, by the right ventricle, it is distributed 
 through the fifth pair of arches over the body, after joining 
 the more aerated blood passing through the fourth pair of 
 arches. 
 
 The blood from the lungs is brought back by two small 
 veins into the left auricle. 
 
 The characteristics of the circulation at this time, are that 
 the blood is aerated by the allantois, and that there is a 
 partial double circulation. (Vide Chap. VII. 9.) 
 
 As soon as respiration commences, the canals leading to 
 the dorsal aorta from the fifth pair of arches, which com- 
 municate only with the right ventricle, become closed. The 
 blood passing along the fifth arch now flows only into the 
 lungs, through the pulmonary arteries. The blood from the 
 left ventricle owing to the cessation of the circulation of the 
 yolk-sac and the allantois is distributed exclusively to the 
 body of the chick, from whence it is all brought back into 
 the right auricle by the three now independently opening 
 venae cavse. 
 
 The portal veins henceforward receive blood from the intes- 
 tines only, and the ductus venosus is obliterated, so that 
 all the blood of the portal vein passes through the capil- 
 laries of the liver. 
 
 The partition between the auricles is rendered complete 
 by the closure of the foramen ovale ; into the right auricle 
 the veins of the body enter, and into the left the pulmonary 
 veins. 
 
 There is thus a completely double circulation formed, in 
 which all the blood of the left ventricle is arterial, and all the 
 blood of the right ventricle venous, and there is at no part 
 of the circulation a mixture of venous and arterial blood. 
 
 12. As early as the sixth day, movements, as we have 
 said, may be seen in the limbs of the embryo upon opening 
 the egg. We may conclude that after this epoch spontaneous 
 movements occur from time to time in the unopened egg. 
 They cannot however be of any great extent until the four- 
 teenth day, for up to this time the embryo retains the position 
 
224 THE SIXTH DAY. [CHAP. VIII. 
 
 in which it was first formed, viz. with its body at right angles 
 to the long axis of the egg. 
 
 On the fourteenth day a definite change of position takes 
 place; the chick moves so as to lie lengthways in the egg, 
 with its beak touching the chorion and shell membrane where 
 they form the inner wall of the rapidly increasing air- 
 chamber at the broad end (Chap. I. 2). 
 
 On the twentieth day or thereabouts, the beak is thrust 
 through these membranes, and the bird begins to breathe the 
 air contained in the chamber. Thereupon the pulmonary 
 circulation becomes functionally active, and at the same time 
 blood ceases to flow through the umbilical arteries. The 
 allantois shrivels up, the umbilicus becomes completely 
 closed, and the chick piercing the shell at the broad end of 
 the egg with repeated blows of its beak, casts off the dried 
 remains of allantois, amnion and chorion, and steps out into 
 the world. 
 
CHAPTER IX. 
 
 THE DEVELOPMENT OF THE SKULL. 
 
 1. IN the chapter on the fifth day, we gave a short 
 description of the earliest stages of the development of the 
 skull. The subject is however of sufficient importance to 
 merit a separate chapter, and in order to render the present 
 account complete in itself, we have found it necessary to 
 repeat a few of the statements already made. 
 
 2. In its earliest condition the cranium is composed of a 
 mesoblastic tissue of stellate cells which can be distinguished 
 from the remainder of the mesoblast by its greater opacity. 
 In this condition (which is that of the fourth day), it may be 
 spoken of as the membranous cranium. From this mem- 
 branous condition the tissue composing it rapidly passes into 
 true hyaline cartilage. 
 
 3. The primitive skull 1 , from its very first formation on 
 the fourth day, consists of elements which fall into two very 
 distinct divisions. We have on the one hand a sheet of 
 cartilage which ensheaths the notochord from its anterior 
 end to the first vertebra. This sheet of cartilage forms an 
 unsegmented continuation of the vertebral bodies. It is to 
 be considered as the most anterior portion of the axial 
 skeleton, in which the segmentation has become obliterated ; 
 
 j and as such is equivalent not to one, but to a (hitherto not 
 certainly determined) number of vertebrae. 
 
 This sheet was spoken of by Rathke (Entwickelungs- 
 geschichte der Natter), its discoverer, as the 'investing mass' 
 
 1 The facts narrated in this chapter are mainly derived from Mr Parker's 
 I Memoir upon the Development of the Skull of the Common Fowl (Gallus 
 k domesticus), Phil. Trans., 1866, Vol. CLVI., pt. i. 
 
 E. 15 
 
226 THE DEVELOPMENT OF THE SKULL. [CHAP. 
 
 from its relations to the notochord, and as such we shall 
 continue to speak of ity 
 
 The second of the two divisions into which the parts of 
 the skull fall, consists of a series of paired rods, whose 
 proximal extremities! are attached more or less closely to the 
 investing mass. All of these (with the exception of the 
 trabeculse) are developed along the axes of the visceral 
 arches. 
 
 FIG. 68. 
 
 *-cv 1 
 
 VIEW FROM ABOVE OF THE INVESTING MASS AND OF THE TRABECUL^ ON 
 
 THE FOURTH DAY OF INCUBATION. (From Parker.) 
 
 In order to shew this the whole of the upper portion of the head has been 
 sliced away. The cartilaginous portions of the skull are marked with the dark 
 horizontal shading. 
 
 cv I cerebral vesicles (sliced off), e eye. nc notochord. iv investing mass. 
 9 foramen for the exit of the ninth nerve, cl cochlea, ksc horizontal 
 semicircular canal, q quadrate. 5 notch for the passage of the fifth nerve. 
 lg expanded anterior end of the investing mass, pts pituitary space. 
 tr trabeculse. The reference line tr has accidentally been made to end a 
 little short of the cartilage. 
 
 We shall commence by describing the ' investing mass * 
 as we find it on the fourth day, and from this pass on to 
 the paired rods attached to it. 
 
IX.] THE INVESTING MASS. 227 
 
 4. The investing mass (Fig. 68, iv) is on the fourth day 
 a broad plate of tissue ensheathing the notochord and arching 
 upwards on each side and especially behind. Laterally it 
 encloses the auditory sacs, and the tissue surrounding these 
 (forming the so-called 'periotic capsules') is in the chick never 
 separate from the investing mass. In front it becomes 
 narrowed, and at the same time excavated so as to form 
 a notch on each side (Fig. 68, 5) through which the fifth 
 nerve passes ; and in front of this again it becomes expanded. 
 
 In order to render our subsequent account more intelligi- 
 ble, we may shortly anticipate the fate of the investing mass. 
 Behind it grows upwards, and the two outgrowths meet 
 above so as completely to enclose the medulla oblongata, 
 and to circumscribe a hole known as the 'occipital foramen' 
 And it is at this point only that the roof of the skull is at 
 any period formed of cartilage. 
 
 Later than this there appear in the investing mass a 
 series of ossifications forming the whole of the occipital bone, 
 and the skeleton of the ear. Knowing this, we shall be able 
 to speak of the different parts of the investing mass, as the 
 regions of the various bones ' ex-occipital/ f basi-occipital' etc. 
 even before the ossifications, which mark these out, have 
 appeared. 
 
 5. In considering the paired rods of cartilage attached to 
 the investing mass, we will commence with the most an- 
 terior. These are known as the 'trabeculce cranii (Fig. 68, tr). 
 They are two narrow rods whose proximal ends are attached 
 to the front end of the investing mass, of which they appear 
 like a forward continuation, while distally they unite with 
 each other. There is thus left an oval (or more nearly 
 circular) space where the cartilage forming the base of the 
 skull is deficient. This space is enclosed behind by the in- 
 vesting mass, and on the two sides and in front by the trabe- 
 cute; it is called the 'pituitary space' (Fig. 68, pts), and in 
 it lies the 'pituitary body/ 
 
 Where the trabeculae unite in front, they form a some- 
 what expanded plate of cartilage continued anteriorly into 
 two diverging horns, which subsequently develope into the 
 all-nasal cartilages. Owing to the cranial flexure the trabe- 
 culge at first lie in a different plane from the investing mass. 
 
 In function, the trabeculse seem almost to serve as a con- 
 
 152 
 
228 THE DEVELOPMENT OF THE SKULL. [CHAP. 
 
 tinuation of the base of the skull, and such they were con- 
 sidered to be by Rathke, their discoverer. Their different 
 mode of development, and (in the lower vertebrates) primitive 
 independence of the investing mass, prove that this is not the 
 case, and that they are almost undoubtedly to be looked on 
 as paired appendages. Their primitive independence of the 
 investing mass is clearly shewn in the skulls of the Marsipo- 
 branchii, in which the trabecute are formed of a dense 
 fibrous tissue and the investing mass of hyaline cartilage. 
 (W. Muller, Ban der Hypophysis u. des Processus infundibuli 
 cerebri. Jenaische Zeitschrift, Vol. vi.) A very probable 
 view, first put forward by Huxley in his Hunterian lectures 
 (vide Anat. Vertebrates, p. 77), and subsequently adopted by 
 Parker as well as other investigators, is, that they are homo- 
 logous with the rods of cartilage developed in the visceral 
 arches, and that they are therefore the remnants of an 
 anterior pair of arches. 
 
 Gegenbaur (Vergleichende Anatomic der Wirbelthiere, in. Heft) looks upon 
 the trabeculse and their coalesced extremities simply as a prevertebral portion 
 of the cranium, and not as paired appendages of the investing mass. Their 
 paired condition seems to militate against this view ; it must be admitted how- 
 ever that our present knowledge of them does not permit us to state with 
 certainty more than that they are not morphologically a continuation of the 
 investing mass. 
 
 In the bird and apparently many higher vertebrates they 
 are never independent of the investing mass, and it was this 
 fact which led to the erroneous view of their nature, formerly 
 adopted, that they were forward continuations of the axial 
 skeleton. 
 
 In them and in the plate of cartilage formed by their 
 coalescence in front appear the ossifications of the whole of 
 the sphenoid, the ethmoid and nasal regions. 
 
 6. The remainder of the paired series of appendages 
 are developed in the visceral arches. The foremost of 
 these are the cartilaginous rods developed in the first visceral 
 or mandibular arch. In our account of the face we men- 
 tion that the mandibular arch on each side produces a 
 bud known as the superior maxillary process, which goes 
 to form the superior boundary of the mouth. In this 
 process, as well as in the primitive arch, rods of cartilage 
 appear. 
 
 The segmentation, so to speak, of the first visceral arch 
 
IX.] THE MAXILL1KY PROCESS. 229 
 
 appears when taken by itself somewhat remarkable, and led 
 to the view, formerly very general and still held by many, 
 that the superior maxillary process was equivalent to a 
 visceral arch. Mr Parker has recently shewn that in both 
 Osseous fishes and the Elasmobranchii all the anterior arches 
 undergo a somewhat similar segmentation, and it appears 
 therefore to be a fair deduction that this segmentation 
 persists in the chick in the mandibular arch, while it has 
 been lost in the others. None of Mr Parker's investigations 
 on the lower vertebrates support the view that the maxillary 
 process is an aborted anterior arch. Similar views as to the 
 nature of the maxillary process have been arrived at by 
 Gegenbaur (loc. cit.}, from his investigations upon the crania 
 of the cartilaginous Fishes. 
 
 7. The cartilage or differentiated mesoblast appears in 
 the maxillary process later than in the mandibular arch, 
 
 FIG. 69. 
 
 VIEW FBOM BELOW OP THE PAIRED APPENDAGES OF THE SKULL OP A FOWL 
 
 ON THE FOURTH DAT OP INCUBATION. (From Parker.) 
 
 cv i cerebral vesicles, e eye. fn fron to-nasal process, n nasal pit. r trabeculae. 
 pts pituitary space, mr superior maxillary process, pg pterygoid. pa pala- 
 tine, q quadrate, mk Meckel's cartilage, ch cerato-hyal. bh basi-hyal. 
 cbr ceratobranchial. ebr proximal portion of the cartilage in the third 
 visceral arch. 66r. basibranchial. i first visceral cleft. 2 second visceral cleft, 
 3 third visceral arch. 
 
230 THE DEVELOPMENT OF THE SKULL. [CHAP. 
 
 and consists of a rod on each side. Each of these early 
 becomes divided into two parts, a proximal and a distal. 
 From the bones which subsequently develope in them the 
 proximal one is known as the pterygoid (Fig. 69, pg\ and 
 the distal one as the palatine (Fig. 69, pa). Both of the 
 rods are very delicate, and remain in the cartilaginous con- 
 dition for a short time only. 
 
 8. In the mandibular arch itself, there is also a proxi- 
 mal and distal cartilage. The proximal cartilage is situated 
 (Figs. 68 and 69, q) at the side of the investing mass but 
 not united with it. It is known as the quadrate, and in the 
 early stage is merely a small knob of cartilage. The distal 
 rod is called MeckeTs cartilage (Fig. 69, mfy ; on it are sub- 
 sequently moulded the bones which form the mandible; 
 while its proximal end becomes the articulare. 
 
 9. In the next arch, usually called the second visceral 
 or hyoid arch, there is a very small development of cartilage. 
 This consists of a central azygos piece the 'basi-hyaV (Fig. 69, 
 bh), and two rods, one on each side, the ' cerato-hyals' (Fig. 69, 
 ch). 
 
 10. In the third arch, which corresponds with the first 
 branchial arch of the Ichthyopsida, there is on each side a 
 large distal cartilaginous rod (Fig. 69, cbr) the ' cerato- 
 branchiall and a smaller proximal piece (Fig. 69, ebr) ; be- 
 tween the two arches lies an undefined mass (Fig. 69, bbr) 
 the ' basibranchial? In the arches behind this one there is 
 in the bird no development of cartilage. 
 
 11. The growth of this primordial cartilaginous skull is 
 very rapid, and by the 5th or 6th day very important changes 
 have taken place. In the first place the pre-pituitary portion 
 of the cranio-facial axis becomes equal to and subsequently 
 actually surpasses in length the post-pituitary part. This 
 change is accompanied by a considerable decrease in the 
 cranial flexure. In the investing mass, the chief changes 
 are an upward growth on each side behind, to form the exoc- 
 cipitals ; and the appearance of the occipital condyles (Fig. 
 70, oc) as small swellings on each side of the middle line 
 at the hind end of the plate. In front of the termination 
 of the notochord two transverse vertical walls of cartilage 
 rise up, the one in front of and the other behind the pitui- 
 tary body: these are called respectively the anterior and 
 
IX.] THE ETHMO-PRESPHENOID PLATE. 231 
 
 posterior clinoid walls. But it is in front of the foremost of 
 these that the most noticeable changes take place. From 
 the mid-line of the coalesced trabeculsB there rises up a 
 high ridge, the ethmo-presphenoid plate. This plate is at first 
 highest behind. In front and below it sends out a process, 
 the prenasal cartilage, which forms the cartilaginous basis 
 on which the premaxillary region is moulded. 
 
 12. Development continues to be very rapid in these 
 parts; and on the seventh day the anterior end of the 
 ethmo-presphenoid plate (Fig. 70, eth and ps) becomes its 
 highest point and forms the retral spike of the ethmoid 
 (Fig. 70, eth). 
 
 FIG. 70. 
 
 bbr 
 
 SIDE VIEW OF THE CARTILAGINOUS CRANIUM OF A FOWL ON THE SEVENTH 
 DAY OF INCUBATION. (From Parker.) 
 
 pn prenasal cartilage, din alinasal cartilage, ale aliethmoid ; immediately 
 below this is the aliseptal cartilage, eth ethmoid, pp pars plana. ps pre- 
 sphenoid. pa palatine, pg pterygoid. , z optic nerve, as alisphenoid. 
 q quadrate, st stapes, fr fenestra rotundum. hso horizontal semicircular 
 canal, psc posterior vertical semicircular canal : both the anterior and the 
 posterior semicircular canals are seen shining through the cartilage, 
 so supraoccipital. eo exoccipital. oc occipital condyle. nc notochord. 
 mk Meckel's cartilage, ch cerato-hyal. bh basi-hyal. cbr and ebr cerato- 
 branchial. bbr basibranchial. 
 
 The prenasal cartilage (Fig. 70, pn) still points down- 
 wards, and by this time are formed the alinasal cartilages 
 (Fig. 70, alri) developed from the trabecular horns, and th 
 aliseptal cartilages which enclose the inferior turbinals 
 (Fig. 70). 
 
 The basisphenoid grows outwards on each side to form 
 
232 THE DEVELOPMENT OF THE SKULL. [CHAP. 
 
 the alisphenoid (Fig. 70, as) and posteriorly the supraoccipitals 
 (Fig. 70, so) have approached much nearer to each other above ; 
 and at the same time the thickenings to form the occipital 
 condyles have greatly increased (Fig. 70, oc). 
 
 The quadrate (Fig. 70, q) has undergone great modifica- 
 tions. In the earlier stage it was a simple knob of cartilage, 
 but now it sends a process forwards, the orbital process, and 
 a long process backwards which articulates with both the 
 'periotic capsule' and the exoccipital (Fig. 70, eo)\ down- 
 wards it sends a two-headed process to articulate with the 
 extremity of Meckel's cartilage. 
 
 The fenestra ovale and fenestra rotundum (Fig. 70, fr) 
 appear during this stage, and in the former the head of the 
 small stapes (Fig. 70, st) is placed. 
 
 The palatine (Fig. 70, pa) and pterygoid (Fig. 70, pg) 
 bars have increased in length, the former being the longer; 
 between them and the base of the skull there has appeared 
 a mass of tissue which will eventually become the sphenoidal 
 rostrum (parasphenoid). 
 
 The second arch is not changed much; while the parts 
 of the third arch, corresponding with the first branchial arch 
 of osseous fishes, have increased in size, but have not under- 
 gone other modifications. 
 
 13. In the next (Parker's third) stage, which occurs 
 about the middle of the 2nd week of incubation, the pre- 
 nasal cartilage (Fig. 71, pri) has greatly increased in length 
 and lost its downward curvature, so that it is in the same 
 straight line as the septum nasi and the ethmoid; the con- 
 tinuous plate thus formed while increasing in length is at 
 the same time narrowed except just in front of the pitui- 
 tary space, where it becomes expanded on each side into an 
 ear-shaped process. 
 
 The pituitary space is still open and admits the internal 
 carotids; behind it is another azygos slit (Fig. 71, nc) along 
 the median line, in which the notochord can be seen. This slit 
 is a new formation, and as the skull outgrows the notochord, 
 the surrounding parts will become the post-pituitary part of 
 the basisphenoid. 
 
 The outgrowths forming the alisphenoids have largely 
 increased. When looked at from below they are almost 
 entirely concealed by the ossified basitemporals (Fig. 71, bf) t 
 
IX.] THE OSSIFICATIONS OF THE CRANIUM. 233 
 
 but their anterior corners just project out in front of it 
 (Fig. 71, CM). ^ 
 
 The occipital condyle formed by the coalescence of the 
 two originally separate cartilaginous knobs, is seen just below 
 the notochord (Fig. 71, no), and the supraoccipitals (Fig. 
 71, so) have coalesced above the occipital foramen. 
 
 14. The ossifications of the cartilaginous skull, which 
 have commenced at this stage, are 1 : 
 
 (1) An ectosteal ossification around the notochord on 
 the inner surface of the skull immediately behind the 
 occipital condyle. This soon spreads to the opposite surface 
 and forms an ossifying centre for the basioccipital. 
 
 (2) An ectosteal ossification (Fig. 71, eo) in each exoc- 
 cipital beginning immediately behind the vagus foramen 
 (Fig. 71, 8) ; this ossification commences on the exterior, but 
 soon spreads round to the inner surface of the cartilaginous 
 exoccipital. 
 
 (3) The palatine (Fig. 71, pa) has become entirely 
 ossified by endostosis, the only instance of this process in the 
 ossification of the primordial skull of the bird. 
 
 (4) An ectostosis of the pterygoid (Fig. 71, pg)* 
 
 (5) An ectosteal ossification of the quadrate (Fig. 71, q). 
 
 (6) An ectosteal ossification of the cerato-branchials. 
 
 15. At this stage the majority of the true membrane 
 bones also begin to be formed. 
 
 The paired premaxillaries (Fig. 71, px) are formed in 
 the investment of the prenasal cartilage ; they are trian- 
 gular with the apex directed forwards, and soon begin to 
 develope their three normal processes : one above towards 
 the frontal (the nasal process), one along the margin of the 
 beak to join the maxillary (the marginal process), and one 
 below along the middle of the palate, to join the palatine 
 bones. The maxillaries (Fig. 71, mx) are developed on 
 each side outside the endoskeleton. Pointed at each end, 
 they are broader in the middle, sending a process inwards 
 
 1 The term ectostosis or ectosteal is used when ossification sets in between the 
 perichondrium and the cartilage, endostosis when ossification takes place be- 
 tween the actual cells of the cartilage. These two processes not unfrequently 
 occur combined, when ossification by ectostosis on reaching the cartilage sets 
 up a true endosteal ossification. Parostosis is used for all ossifications which 
 take place in purely fibrous tracts. Parosteal products are often spoken of 
 as membrane or splint bones. 
 
234 
 
 THE DEVELOPMENT OF THE SKULL. 
 FIG. 71. 
 
 [CHAP. 
 
 rbs 
 
 EMBRYONIC SKULL OP A FOWL DURING THE SECOND WEEK OP INCUBATION 
 
 (THIRD STAGE) FROM BELOW. (From Parker. ) 
 
 The figure is intended to shew the cartilaginous skull with its ossifications, 
 and also the splint bones which have commenced to be formed independently of 
 the cartilaginous cranium. The parts of the lower jaw and hyoid are omitted. 
 The following letters refer to the cartilaginous cranium and its ossifications. 
 pn prenasal cartilage, pa palatine bone (which has already become ossified). 
 pg pterygoid (also ossified), nc notochord visible in the post-pituitary space. 
 Both the notochord and the post-pituibary space would be in reality per- 
 fectly concealed by the plate of bone bt, but have been diagrammatically re- 
 presented as if they were visible through it. nc end of notochord appearing 
 beyond the occipital condyle. q quadrate, as alisphenoid. ic internal 
 carotid, ty tympanic cavity, psc posterior semicircular canal. 8 foramen for 
 passage of eighth nerve. 9 foramen for passage of ninth nerve, eo centre of 
 ossification for the occipital, so supraoccipital region, fm foramen magnum. 
 The following letters refer to the splint bones. 
 
 px premaxillary. mx maxillary, pmx process inwards from the premaxillary to 
 form the maxillo-palatine. j jugal. qj quadrato-jugal. rbs sphenoidal 
 rostrum, bt basitemporal. '.,; 
 
IX.] THE PARASPHENOID. 235 
 
 to form the maxillo-palatine (Fig. 71, pmx). The jugal 
 (Fig. 71, j) and the quadrato-jugals (Fig. 71, qj) are also 
 developed during this stage, and connect the hinder end of 
 the maxillaries with the quadrate ; they are both delicate 
 rod-like bones, the quadrato-jugal being the larger. 
 
 The top of the cranium, which up to this* time has not 
 been covered by either cartilage or bone, begins now to be 
 closed in by parosteal osseous deposits taking place to form 
 the nasals, frontals, and parietals. Outside the prootic 
 region the squamosal also begins to be formed by a deposit 
 taking place in the mesoblast external to the perichondrium. 
 
 In the base of the skull, there are three osseous deposits 
 to form membrane bones, which soon uniting constitute the 
 equivalent of the parasphenoid of osseous fishes. 
 
 The first of these is the osseous deposit which takes place 
 in the mass of tissue, previously spoken of as the rostrum 
 (Fig. 71, rbs); this is connected behind with two osseous 
 deposits, the basisphenoidal ossicles, formed one on each side 
 in front of the pituitary space. The third deposit is the 
 large somewhat reniform mass of bone (Fig. 71, bt) called 
 the basitemporal, which forms one of the most conspicuous 
 features in the skull at this stage, and consists of two sym- 
 metrical halves. The basitemporals have not at this stage a 
 very firm connection with the base of the skull, but shortly 
 they become firmly united with it and with one another, and 
 thus the pituitary space becomes entirely closed over below 
 by bone. 
 
 16. During the previous stage, most of the bones of the 
 face and skull had begun to be ossified, so that during the 
 next or fourth stage, that is about the end of the second and 
 the beginning of the third week of incubation, the chief 
 changes consist in the progressive ossification of the various 
 bones. Since it is not our purpose to enter fully into the 
 structure of the skull of the fowl but rather to confine our- 
 selves to its early development, this and the succeeding 
 stages will be described with greater brevity than the first 
 three. 
 
 17. One most important change which occurs during this 
 stage, is the fenestration of the ethmo-presphenoid cartilage. 
 This cartilage (Fig. 70, ps) formed during the second stage 
 an unbroken plate, but towards the end of the third and 
 
236 THE DEVELOPMENT OF THE SKULL. [CHAP. 
 
 the beginning of the fourth stages, two fenestrae appear. 
 The hindermost of these separates the presphenoid from the 
 ethmoid ; but since the hole does not extend to the bottom 
 of the plate, the basi-sphenoid and the ethmoid are continu- 
 ous below. The other fenestra separates the ethmoid from 
 the nasal septum. 
 
 The early condition of the ethmoido-nasal cartilage re- 
 sembles, as Parker points out, its permanent condition in the 
 Struthiadce, while the condition which it acquires during this 
 stage resembles its condition in Tinamine birds, seeming to 
 prove that the Gallinaceous birds passed through these two 
 stages before reaching the present condition. This plate of 
 cartilage remains during this stage quite unossified. The 
 aliethmoid, aliseptal, and alinasal cartilages which grow out 
 from it on each side, are now severally separated from each 
 other by slight grooves. 
 
 18. The chief new ossifying centres which appear in the 
 primordial skull in this stage are : 
 
 (1) An ectosteal plate for the prootic. 
 
 (2) Two ectosteal plates which appear in each ali- 
 sphenoid, one in the upper corner, and another immediately 
 above the foramen ovale. 
 
 The centres which had begun during the last stages have 
 now spread considerably. 
 
 The basioccipital is to a great extent ossified, but still 
 encloses the remains of the notochord. The occipital condyle 
 is however still unossified. Both the exoccipital and supra- 
 occipital plates are spreading rapidly, and the latter have 
 nearly reached the middle line. The quadrate is nearly 
 entirely ossified, but its upper and lower condyles and orbital 
 process are still cartilaginous. 
 
 19. The rostrum is still separate from the ethmoid, 
 but has to a great extent coalesced with the basisphe- 
 noid. Between the posterior end of the pterygoid and the 
 rostrum, a plate of cartilage is interposed, called the basi- 
 pterygoid. 
 
 The basitemporals have coalesced to a great extent with 
 the osseous outgrowths of the basisphenoid, but at the sides 
 a space is left between the two, through which the Eusta- 
 chian tubes pass. Between the basitemporals themselves, 
 where they have not coalesced, another space is left, which 
 
IX.] THE OSSIFICATIONS OF THE EAR. 237 
 
 becomes part of the common vestibule in which the Eusta- 
 chian tubes meet in the middle line. 
 
 One new splint bone, the vomer } begins to be formed 
 during this stage in the middle line about half way along 
 the palatines. 
 
 The various membrane-bones of the skull are now much 
 more firm and consistent, and the frontals have sent down a 
 process which upon reaching the ethmoidal plate at once 
 sets up ossification in it. 
 
 20. In the next stage, about the second day after birth, 
 a considerable number of changes have taken place. The 
 ethmoid has begun to be ossified by two ectosteal plates, one 
 on each side, and the ossification set up by the frontal process 
 in the ethmoid plate has increased. The ethmoid is now only 
 connected with the septum nasi by a narrow isthmus above. 
 
 The septum still retains its anterior process, the pre- 
 nasal cartilage, but this is rapidly diminishing. 
 
 In the ear there are now three ectosteal ossifications, the 
 prootic, which appeared in the last stage, and which is by far 
 the largest of the three ; the opisthotic, which lies between 
 the prootic and the exoccipital, but is distinct from both ; and 
 a third small ectosteal plate, the pterotic. 
 
 A small ring-like ossification appears in the internal 
 angular process of Meckel's cartilage ; its other process 
 rapidly diminishes and soon disappears. 
 
 The supraoccipital ossifications have united in the 
 middle line ; the exoccipitals have increased very much, but 
 are quite separate from the opisthotics. 
 
 The basioccipital ossification has begun to spread into 
 the condyles. 
 
 21. The chief facts of importance in reference to the 
 splint bones, are, that the premaxillaries (Fig. 71, px) have 
 united in the middle line ; the vomer has considerably 
 increased in size; the lacrymals have developed, and 
 possess a large supraorbital and a thick anteorbital plate. 
 There is still a large fontanelle above, between the frontals 
 and parietals. The squamosals are very large. In the man- 
 dible all the splint bones have by this time become developed. 
 
 At about the third week after birth two new centres 
 appear, one at the outside of the alisphenoid, forming the 
 centre for the post-frontal, and another over the posterior 
 
238 THE DEVELOPMENT OF THE SKULL. [CHAP. IX. 
 
 semicircular canal, for the epiotic, which is for a short time 
 distinct from the exoccipital. 
 
 In chicks of about two months old, the sutures have begun 
 to lose their distinctness, but two new pairs of centres of 
 ossification appear adjacent to the presphenoid. These first 
 ossify independently, but afterwards set up ossification in the 
 presphenoid, which is still unossified. In fowls of from seven to 
 nine months, the splint bones are beginning to coalesce for the 
 most part with the other bones of the skull, and the sutures 
 are rapidly being obliterated. It would be beyond the scope 
 of this work to enter into the changes by which the skull of 
 the young chick, with most of the sutures distinct, developes 
 into the skull of the adult, where the lines of junction of 
 most of the bones are quite undistinguishable. We shall 
 therefore conclude by giving a table of those bones which 
 are preformed in cartilage, and of the purely splint or 
 membrane bones. 
 
 Parts of the birds skull which are either preformed in 
 cartilage or remain cartilaginous. 
 
 Supraoccipital. Exoccipital. Basioccipital. Epiotic. 
 Prootic. Opisthotic. Pterotic. Alisphenoid. Basisphenoid. 
 Orbitosphenoid. Presphenoid. Ethmoid. Post -frontal. 
 Septum nasi, turbinals, prenasal and nasal cartilages. Skele- 
 ton of the second and third visceral arches and stapes. 
 Meckel's cartilage and quadrate (first visceral arch). Ptery- 
 goid and palatine (superior maxillary process). 
 
 Splint-bones not preformed in cartilage. 
 
 Parietals. Squamosals. Frontals. Lacrymals. Nasals. 
 Premaxillse. Maxillae. Maxillo-palatines. Vomer. Jugals. 
 Quadrato-jugals. Dentary and Bones of Mandible. Basi- 
 temporal and rostrum. 
 
APPENDIX. 
 
 PRACTICAL INSTRUCTIONS FOR STUDYING THE DEVELOPMENT 
 OF THE CHICK. 
 
 I. Incubators. 
 
 OF all incubators, the natural one, i.e. the hen, is by far 
 the best. The number of eggs which fail to develope is much 
 fewer than with an artificial incubator, and the progress of 
 development is much more regular. Under the hen an egg 
 taken, say at 36 hours, will most probably be found in the 
 stage which we have described as proper to that date ; in an 
 artificial incubator, it is nearly sure not to be so. A good 
 sitter will continue to sit for thirty or more days at least, 
 even though the eggs are daily being changed. She should 
 never be allowed to want for water, and should be well 
 supplied according to her appetite with soft food. It is best 
 to place the food at some little distance from the eggs, in 
 order that the hen may leave the eggs when feeding. She 
 will sit most persistently in a warm, quiet, somewhat darkened 
 spot. When an egg is placed under her, the date should be 
 marked on it, in order that the duration of its incubation 
 may be exactly known. When the egg is intended to remain 
 for some time, e.g. for seven days or more, the mark should 
 be bold and distinct, otherwise it will, be rubbed off. 
 
 Where a "broody" hen cannot be obtained, recourse must 
 be had to an artificial incubator. We have ourselves been 
 so accustomed to employ a hen, that we have very little 
 experience of the various machines which have been intro- 
 duced as incubators. We have, however, obtained tolerably 
 satisfactory results with an ordinary chemical double-jacketed 
 drying water-bath, thoroughly covered in with a thick coat of 
 ootton wool and flannel baize, and heated by a very small 
 
240 PRACTICAL DIRECTIONS. [APP. 
 
 gas-jet. If the vessel be filled with hot water, and allowed to 
 cool down to 40 or thereabouts, before the eggs are intro- 
 duced, a very small gas flame will be sufficient to maintain 
 the requisite temperature. A small pin-hole-nozzle, giving 
 with ordinary pressure an exceedingly narrow jet of flame 
 about two inches high, is the most convenient. By turning 
 the gas off or on, so as to reduce or increase the height of the 
 jet as required, a very steady mean temperature may be 
 maintained. A simple arrangement of this kind on the whole 
 works more satisfactorily than any of the complicated instru- 
 ments which have been introduced for similar purposes. 
 
 In the absence of gas, a patent night-light placed at a 
 proper distance below the bath may be made to answer very 
 well. When a body of water, once raised to the necessary tem- 
 perature, is thoroughly surrounded with non-conducting ma- 
 terial, a very slight constant amount of heat will supply all 
 the loss. 
 
 The temperature should be from 37 to 40C. ; it may rise 
 temporarily a few degrees above 40 without any permanent 
 harm, and should not be allowed to fall below 37. 
 
 The eggs within the bath should be placed on and covered 
 up with cotton wool ; and the products of the combustion of 
 the gas should be kept as much from them as possible. 
 
 II. Examination of a 36 to 48 hours embryo. 
 
 The student will find it by far the best plan to begin 
 with the study of an embryo of this date. The manipulation 
 is not difficult ; and the details of structure are sufficiently 
 simple to allow them to be readily grasped. Earlier embryos 
 are troublesome to manage until some experience has been 
 gained ; and the details of later ones are so many as to render 
 it undesirable to begin with them. 
 
 A. Opening the egg. 
 
 Take the egg warm from the hen or the incubator, 
 and place it (it does not matter in what position, 
 since the blastoderm will at this stage always be found 
 at the uppermost part of the egg) in a small basin 
 large enough to allow the egg to be covered with 
 fluid. It is of advantage, but not necessary, to place 
 
PP.] OPENING THE EGG. 241, 
 
 at the bottom of the basin a mould, e.g. a flat piece of 
 lead with a concavity on the upper surface, in which 
 the egg may rest securely without rolling. Pour into 
 the basin so much of a "75 per cent, solution of sodium 
 chloride warmed to 38 as will cover the egg com- 
 pletely. With a sharp tap break through the shell at 
 the broad end over the air-chamber, and let out as 
 much air as has already been gathered there. Unless 
 *: this is done, the presence of air in the air-chamber 
 will cause the broad end to tilt up. At this date 
 there will be very little air, but in eggs of longer 
 incubation, inconvenience will be felt unless this plan 
 be adopted. 
 
 Instead of being broken with a blow, the shell 
 may be filed through at one point, and the opening 
 enlarged with the forceps ; but a little practice will 
 enable the student to use the former and easier 
 method without doing damage. 
 
 With a blunt pair of forceps, remove the shell 
 carefully bit by bit, leaving the shell-membrane 
 behind ; begin at the hole made at the broad end, and 
 work over the upper part until about a third or half 
 of the shell has been removed. 
 
 Then with a finer pair of forceps remove the sheik 
 membrane ; it will readily come away in strips, torn 
 across the long axis of the egg in a somewhat spiral 
 fashion. The yolk and embryo will now come into view. 
 
 It is the practice of some simply to break the egg 
 across and pour the yolk and white together into a 
 basin, very much as the housewife does. We feel sure, 
 however, that the extra trouble of the method we 
 have given will be more than repaid in the results. 
 
 During this time, and indeed during the whole 
 period of the examination of the embryo in situ, the 
 basin and its contents must be maintained, either by 
 renewal of the salt solution, or by the basin being 
 placed on a sand-bath, at about 38C. 
 
 B. Examination of the blastoderm in situ. 
 
 This may be done with the naked eye, or with a 
 simple lens of low power. Observe : 
 E. 16 
 
242 PRACTICAL DIRECTIONS. [APP. 
 
 1. Lying across the long axis of the egg, the pellucid 
 area, in the middle of which the embryo may be 
 obscurely seen as a white streak. 
 
 2. The mottled vascular area, with the blood-vessels 
 just beginning to be formed. 
 
 3. The opaque area spreading over the yolk with the 
 changes in the yolk around its periphery. 
 
 4. (With a simple lens), the contractions of the heart; 
 perhaps the outlines of the head of the embryo may 
 be detected. 
 
 C. Removal of the embryo. 
 
 Plunge one blade of a sharp fine pair of scissors 
 through the blastoderm, just outside the outer margin 
 of the vascular area, and rapidly carry the incision 
 completely round. Until the circle is complete, avoid 
 as much as possible any agitation of the liquid in the 
 basin. 
 
 With a little trouble, the excised blastoderm may 
 now be floated into a watch-glass, care being taken to 
 keep it as flat as possible. With a pair of forceps or 
 with a needle, aided by gentle shaking, remove the 
 piece of vitelline membrane covering the blastoderm. 
 
 If any yolk adheres to the blastoderm, it may with 
 a little gentle agitation easily be washed off. Some- 
 times it is of advantage to suck up the yolk with a 
 glass syringe, replacing the fluid removed with clean 
 (*75 p. c.) salt solution. 
 
 The blastoderm should now be removed from the 
 watch-glass to a microscopic glass slide ; since it is 
 difficult in the former to prevent the edges of the 
 blastoderm from curling up. 
 
 The transference may easity be effected, if both the 
 watch-glass and slide are plunged into a basin of clean 
 warm salt solution. With a little care, the blasto- 
 derm can then be floated from the one to the other, and 
 the glass slide, having the blastoderm with its upper 
 surface uppermost spread flat upon it, very gently 
 raised out of the liquid. 
 
APP.] SURFACE VIEW OF EMBRYO. 243 
 
 A thin ring of putty may now be placed round the 
 blastoderm, a small quantity of salt solution gently 
 poured within the ring, and the whole covered with a 
 glass slide, which may be pressed down until it is 
 sufficiently close to the embryo. The presence of any 
 air-bubbles must of course be avoided. 
 
 Provided care be otherwise taken to keep the 
 embryo well covered with liquid, the putty ring and 
 the coverslip may be dispensed with. They are often 
 inconvenient, as when the embryo has to be turned 
 upside down. 
 
 The object is now ready for examination with a 
 simple lens or with a compound microscope of low 
 objective. It is by far the best for the student to 
 begin at least with the simple lens. In order that 
 everything may be seen at its best, the slide should be 
 kept warmed to about 38, by being placed on a 
 hot stage. 
 
 D. S^l,rface view of the transparent embryo from 
 
 above. 
 
 The chief points to be observed, are : 
 
 1. The head-fold. 
 
 2. The indications of the amnion ; especially the false 
 amnion, or outer amniotic fold. 
 
 3. The neural tube : the line of coalescence of the 
 medullary folds, the first cerebral vesicle, the com- 
 mencing optic vesicles, the indications of the second 
 and third cerebral vesicles, the as yet open me- 
 dullary folds at the tail end. 
 
 4. The heart seen dimly through the neural tube ; note 
 its pulsation if present. 
 
 5. The fold of the somatopleure anterior to the heart 
 (generally very faintly shewn). 
 
 6. The fold of the splanchnopleure (more distinctly 
 seen) : the omphalo-mesaraic veins. 
 
 7. The protovertebrce. 
 
 162 
 
244? PRACTICAL DIRECTIONS. [APP. 
 
 8. Indications of the omphalo-mesaraic arteries. 
 
 9. The as yet barely formed tail-fold. 
 
 10. The commencing blood-vessels in the pellucid and 
 vascular areas. 
 
 E. Surface view of the transparent embryo from 
 
 below. 
 
 The coverslip must now be removed and the glass 
 slide again immersed in a vessel of clean salt solution. 
 By gently seizing the extreme edge of the opaque 
 area with a pair of forceps, no difficulty will be found 
 in so floating the blastoderm, as to turn it upside 
 down, and thus to replace it on the slide with the 
 under surface uppermost. 
 
 The points which most deserve attention in this 
 view, are : 
 
 1. The heart : its position, its union with the omphalo- 
 mesaraic veins, its arterial end. 
 
 2. The fold of the splanchnopleure marking the hind 
 limit of the gut ; the omphalo-mesaraic veins run- 
 ning along its wings. 
 
 3. The protovertebrce on each side of the neural canal 
 behind the heart ; farther back still, the vertebral 
 plates not divided into protovertebrse. 
 
 F. The examination of the embryo as an opaque 
 
 object. 
 
 This should never be omitted. Many points in 
 the transparent embryo only become intelligible after 
 the examination of it as an opaque object. 
 
 Having removed the putty ring and coverslip, if 
 previously used, allow the blastoderm so far to become \ 
 dry, that its edge adheres to the glass slide. Care 
 must of course be taken that the embryo itself does 
 not become at all dry. Place the glass slide with the 
 blastoderm extended flat on it, in a shallow vessel 
 containing a '1 or '5 per cent, solution of chromic acid. 
 
.] THE EMBRYO AS AN OPAQUE OBJECT. 245 
 
 If the blastoderm be simply immersed by itself in 
 the chromic acid solution, the edges of the opaque 
 area will curl up and hide much of the embryo. The 
 method suggested above prevents these inconveni- 
 ences. 
 
 The embryo thus hardened and rendered opaque 
 by immersion in chromic acid, (a stay of 24 hours in 
 the solution will be sufficient) may be removed to a 
 ' watch-glass, containing either some of the solution, or 
 plain water, and examined with a simple lens, under 
 a strong direct light. The compound microscope will 
 be found not nearly so advantageous for this purpose 
 as the simple lens. A piece of black paper placed under 
 the watch-glass, will throw up the lights and shadows 
 of the embryo, with benefit. The watch-glass should 
 have a flat bottom; or a shallow flat glass cell should 
 be used instead. 
 
 a. Looking at the embryo from above, observe : 
 
 1. The head-fold; the head distinctly projecting from 
 the plane of the blastoderm, and formed chiefly by 
 the forebrain and optic vesicles. 
 
 2. The elevation of the medullary canal, and the 
 indications of the side walls of the embryo. 
 
 3. The indications of the tail. 
 
 4. The Amnion partly covering the head. Tear it open 
 with needles. Observe its two folds. 
 
 b. Having turned the blastoderm upside down, 
 observe the following points, looking at the embryo 
 from below. 
 
 1. The hinder limit of the splanchnopleure in the head- 
 fold, marking the hind limits of the fore-gut. The 
 opaque folds now conceal the head almost entirely 
 from view. 
 
 2. The commencing tail-fold, and the shallow boat- 
 shaped cavity (of the alimentary canal) between it 
 and the head-fold. * 
 
 The student should not fail to make sketches of 
 
246 PKACTICAL DIRECTIONS. [APP. 
 
 the embryo, both as a transparent, and as an opaque 
 object, seen from below as well as from above. 
 These sketches will be of great service to him when 
 he comes to study the sections of the same embryo. 
 
 G. On preparing sections of the embryo. 
 1. HARDENING. 
 
 a. With chromic acid. 
 
 The embryo must be immersed in the way 
 recommended under F, in a solution of the 
 strength of *1 p. c. for 24 hours. From this it 
 should be removed and placed in a stronger 
 solution (*3 p. c.) for another 24 hours. If it then 
 appears sufficiently hard, it may be at once 
 placed in alcohol of 70 p. c., in which it should 
 remain for one day, and then be transferred to 
 alcohol of 90 p. c., after remaining in which for 
 two days, it should lastly be placed in absolute 
 alcohol ; in this it can stay till required for section. 
 If the chromic acid has not by this time been com- 
 pletely got rid of, the absolute alcohol must be 
 changed, till the specimen is entirely free from 
 acid. 
 
 b. With picric add. 
 
 The best form for applying the picric acid, is 
 that introduced by Dr Kleinenberg. 
 
 With 100 parts of water, make a cold saturated 
 solution of picric acid ; add to this two parts of 
 concentrated sulphuric acid : filter and add to the 
 filtrate three times its bulk of water. 
 
 In this solution of picric acid, the embryo 
 must be immersed in the same manner as when 
 chromic acid is used (vide F). After remain- 
 ing in the acid for Jive hours, it is to be treated 
 successively with weak, strong and absolute alco- 
 hol, as was done in the case of the chromic acid. 
 Still more difficulty will probably be found in 
 getting rid of the picric acid, than was found to 
 
APP.] HARDENING AND STAINING. 247 
 
 be the case with the chromic acid, and the 
 alcohol will probably have to be changed several 
 times. 
 
 c. With osmic acid. 
 
 Immerse the embryo in a '5 p. c. solution of 
 osmic acid ; leave it in this, covered and in the 
 dark, for 2J hours, and then place the embryo in 
 absolute alcohol, taking care completely to get 
 rid of the acid by washing several times with 
 alcohol. 
 
 The osmic acid method has the advantage of 
 being simpler than the others, and also of leaving 
 the cells in a more natural condition. Its dis- 
 advantages are, that it is less certain, and further, 
 that it is necessary to cut sections of the embryo 
 the next day after it has been placed in the spirit. 
 Otherwise it becomes too brittle ; the sections are 
 then not so easy to make, and not so good. It has 
 the further disadvantage that the outlines of the 
 individual cells are not so clearly brought out as 
 with the other two reagents. 
 
 Absolute alcohol has also been employed as a 
 hardening reagent, but is by no means so good 
 as the reagents recommended above. 
 
 2. STAINING. 
 
 In most cases it will be found of advantage to 
 stain the embryo. The best method of doing this, 
 is to stain the embryo as a whole, rather than to 
 stain the individual sections after they have been 
 cut. 
 
 Either carmine or haematoxylin may be em- 
 ployed. For carmine, Beales' or some other alco- 
 holic solution is the best. Into this the embryo 
 may be removed directly from the absolute alcohol, 
 left in the carmine for 24 hours, and then placed 
 again in absolute alcohol for another 24 hours 
 before being cut. 
 
 The best solution of hsematoxylin, one for which 
 
PRACTICAL DIRECTIONS. [API*. 
 
 we are indebted to Dr Kleinenberg, is made in the 
 following way. 
 
 1. Make a saturated solution of crystallized calcium 
 chloride in 70 p. c. alcohol, and add alum to 
 saturation. 
 
 2. Make also a saturated solution of alum in 70 p. c. 
 alcohol, and add 1 to 2 in the proportion of 1:8. 
 
 3. To the mixture of 1 and 2 add a few drops of a 
 'barely alkaline saturated solution of hsematoxy- 
 lin. 
 
 The embryo may be removed into this directly 
 from the absolute alcohol, left in it for 5 hours 
 and placed back again in absolute alcohol for 24 
 hours. 
 
 Neither the carmine nor the haematoxylin will 
 stain if the embryo is not quite free from acid. 
 
 Only the chromic or picric acid specimens re- 
 quire to be stained. It is however possible to stain 
 the osmic acid specimens with hsematoxylin. 
 
 3. IMBEDDING. 
 
 It is hardly possible to obtain satisfactory sec- 
 tions of embryos, without employing the method of 
 imbedding now so largely employed in histological 
 studies. 
 
 The substances most generally used, are: 
 
 1. Paraffin, obtained by heating together five parts 
 of solid paraffin with one part of paraffin oil and 
 one part of pig's lard. 
 
 2. Wax and oil, made by heating together three parts 
 of common white wax and one of olive-oil. 
 
 3. Spermaceti, obtained by heating together very 
 thoroughly four parts of spermaceti and one of 
 cocoa butter, or four parts of spermaceti and one 
 of castor oil. 
 
 [4. Gum. The above substances are all liquid when 
 warm, and become solid when cold. When a 
 solution of gum is used as an imbedding medium, 
 
APP.] IMBEDDING. 249 
 
 it must be hardened with, alcohol. We cannot 
 however recommend this substance for embryos. 
 For the method of employing it, see Hand-book 
 for ike Physiological Laboratory, p. 92.] 
 
 The exact consistency of these various mixtures 
 should be made to vary according to the solidity 
 of the embryo, by increasing or diminishing the 
 various constituents. 
 
 From a cold solid mass of one of the mix- 
 tures, cut a cubical block (about an inch cube), 
 and on one surface make a shallow cavity, large 
 enough to receive the blastoderm. Having with 
 a morsel of blotting-paper removed all superfluous 
 alcohol from the blastoderm place it quite flat 
 in the cavity just made, and gently pour over it 
 some of the mixture sufficiently heated to have 
 become liquid (care being taken, of course, that 
 it is not too hot). With a heated needle, remove 
 any bubbles that may make their appearance ; and 
 make on the block, as a guide in cutting the sections, 
 a mark of the exact position of the embryo. It is 
 sometimes of advantage to transfer the embryo from 
 the alcohol to some oil of cloves (when the wax and 
 oil is used, or to some creosote, when the paraffin is 
 employed), and to allow it to become saturated 
 with that substance, before placing it in the block. 
 The adhesion of the imbedding material to the 
 object imbedded, is thus rendered more complete. 
 
 This method serves fairly well, when there are 
 no cavities in the object into which it is necessary 
 for the mixture to penetrate. When these exist, 
 the following method (again Dr Kleinenberg's) will 
 be found the most satisfactory. 
 
 Remove the object from the absolute alcohol 
 and place it directly into Bergamot oil, and leave it 
 there, till the oil has completely penetrated through 
 it. In the mean time, prepare a small paper box, 
 by bending up the sides and folding in the corners 
 of a piece of stiff paper, and pour on the bottom of 
 it a thin coating of the mixture of spermaceti and 
 castor oil. When this has become solid, place the 
 
250 PRACTICAL DIRECTIONS. [APP, 
 
 embryo flat upon it, having removed as much super- 
 fluous Bergamot oil as possible, and then pour in 
 some more of the spermaceti mixture, which should 
 be as hot as can be used without injuring the object. 
 With a hot needle, move the object about so as to 
 get rid of all air bubbles, and to assist the sperma- 
 ceti in penetrating all parts of the embryo. Finally, 
 before the spermaceti becomes solid, place the 
 embryo in the position required for subsequently 
 making sections ; and when the whole becomes 
 solid, make a mark over the position of the embryo. 
 It is better to soak the object in the hot spermaceti 
 before finally imbedding it, but the manipulation of 
 this is rather difficult with embryos of this age. 
 
 If successfully imbedded, the spermaceti will be 
 found to have penetrated through and through the 
 embryo ; and the method has the great merit of per- 
 mitting the imbedding medium to be quite easily 
 dissolved away from the sections, by a few minutes' 
 immersion in a mixture of four parts of turpentine 
 to one of creosote. With the other imbedding 
 materials of which we have spoken, this cleaning 
 of the sections is very troublesome and difficult, and 
 liable to result in injury to the specimens. 
 
 4. CUTTING SECTIONS. 
 
 When the imbedding block is cold pare away 
 the edges and then gradually slice it away until the 
 edge of the area opaca appears. The sections must 
 then be made more carefully, and each one ex- 
 amined till the actual body of the embryo is arrived 
 at. 
 
 It is best to begin with transverse vertical 
 sections which may commence either at the tail or 
 head end. The latter is preferable, since the bit 
 which is last cut is apt to slip. 
 
 A section instrument may be used ; but we our- 
 selves very much prefer a simple razor provided 
 with heavy fixed handle (i.e. with no hinge) held 
 in the hand. 
 
APR] CUTTING SECTIONS. 251 
 
 In any case the cutting-blade should for the 
 first method of imbedding be kept freely wetted 
 with spirit; and each section, as it is made, carefully 
 floated on to a glass slide and then, the spirit 
 having been removed, mounted in glycerine, or 
 treated with creosote or oil of cloves and turpentine 
 and mounted in balsam or dammar. It is well to 
 guard the section with a small diaphragm of paper 
 placed under the cover-slip. 
 
 When the object has been imbedded in sper- 
 maceti in the way recommended above, the block 
 of spermaceti and the razor should be moistened 
 with olive oil and not with spirit. The sections 
 must be floated from the razor on to the glass 
 slip in the ordinary way and then treated with a 
 mixture of turpentine (4 parts) and creosote (1 
 part) till all the oil and the spermaceti are removed. 
 They may then be mounted in Canada balsam or 
 dammar in the usual way. Even when the other 
 methods of imbedding spoken of above are adopted, 
 it will frequently be found advantageous to moisten 
 the razor with creosote or oil of cloves rather than 
 with spirit. 
 
 Whichever method be followed, a series of sec- 
 tions, each as thin as possible, should be obtained 
 and carefully numbered, from head to tail or vice 
 versa. They should at first at least all be retained 
 for study, and not even the fragmentary ones 
 thrown away, these being often the most instructive. 
 
 The following transverse sections will perhaps 
 be the most instructive. 
 
 1. Through the optic vesicles, shewing the optic stalks. 
 
 2. Through the hind-brain, shewing the auditory sacs. 
 
 3. Through the middle of the heart, shewing its rela- 
 tions to the splanchnopleure and alimentary canal. 
 
 4. Through the point of divergence of the splanchno- 
 pleure folds, shewing the venous roots of the heart. 
 
 5. Through the dorsal region, shewing the medullary 
 
252 PRACTICAL DIRECTIONS. [APP. 
 
 canal, proto vertebrae and commencing cleavage of 
 the mesoblast. 
 
 6. Through a point where the medullary canal is still 
 open, shewing the mode in which its closing takes 
 place. 
 
 A good series of transverse sections having been 
 obtained, longitudinal vertical ones may be made 
 in a similar manner. These are however much 
 more difficult to manage ; and are moreover chiefly 
 of use to compare with the transverse ones. 
 
 GG. Preservation of the embryo as a whole. 
 
 Embryos of this or an earlier day may be easily 
 preserved whole as microscopic objects; but the value 
 of such preparations is very slight, and they are per- 
 haps not worth the trouble. The best method is 
 probably to place the embryos in the picric acid 
 solution for a short time, and then successively in 
 weak, strong and absolute alcohol and finally to mount 
 them, in glycerine or, after staining and treatment 
 with oil of cloves, in balsam. For shewing some 
 points, treatment with osmic acid for a short time and 
 then with alcohol and finally with glycerine may be 
 adopted. 
 
 Whole embryos of a later date cannot be satis- 
 factorily preserved as microscopic objects. 
 
 III. Examination of an Embryo of about 48 50 hours. 
 
 A. Opening the egg as in II. A. 
 
 B. Examination of the blastoderm in situ. 
 Observe 
 
 1. The form of the embryo, which is much more 
 distinct than at the earlier stage. 
 
 2. The beating of the heart. 
 
 3. The general features of the circulation. 
 
APP.] EMBRYO OF THE THIRD DAY. 253 
 
 C. Removal of the embryo from the yolk, as in 
 
 II. C. 
 
 D. Surface view of the transparent embryo from 
 
 above. 
 
 Notice: 
 
 1. General form of the embryo. 
 
 a. Commencing cranial flexure. 
 
 b. The tail and side folds. 
 
 2. Amnion. Notice the inner and outer (false amnion) 
 limbs and remove them with a needle. When the 
 amnion has been removed the features of the 
 embryo will be much more clearly visible. 
 
 3. The organs of sense. 
 
 a. Eye. Formation of the lens already nearly 
 completed. 
 
 b. Auditory involution, now a deep sac with a 
 narrow opening to the exterior. 
 
 4. The brain. 
 
 a. The vesicles of the fore-, mid~, and Amc?-brain. 
 
 b. The two cerebral lobes, which have been budded 
 off from the fore-brain. 
 
 c. The cranial flexure taking place at the mid- 
 brain. 
 
 E. Transparent embryo from below. 
 Manipulation as in II. E. 
 
 Notice : 
 
 1. The increase of the head-folds of the somatopletire 
 and splanchnopleure, especially the latter, and the 
 commencement of these folds at the tail. 
 
254 , PRACTICAL DIRECTIONS. [APP. 
 
 2. The now GC-shaped heart; for further particulars 
 vide Chap. iv. 18. 
 
 3. The commencing 1st and 2nd visceral clefts and 
 the aortic arches. 
 
 4. The circulation of the yolk sac, vide Fig. 23. Make 
 out all the points there shewn and ascertain 
 by examination that what have been called the 
 veins and arteries in that figure, are truly such. 
 
 F. The embryo as an opaque object. 
 Treatment as in II. F. 
 
 FROM ABOVE : 
 
 Observe the amnion, which is a very conspicuous 
 object, and remove it with needles if not done pre- 
 viously. The external form of the brain and the 
 auditory sac appear very distinctly. 
 
 FROM BELOW: 
 
 Observe the nature of the head- and tail-folds, 
 which are much more easily understood from the 
 opaque than from the transparent embryos. 
 
 Observe also the alimentary canal, the widely open 
 hind end of the fore-gut, and the front end of the as 
 yet very short hind-gut. 
 
 G. Sections. 
 
 Manipulation as in II. G. The embryo can still 
 be stained as a whole. 
 
 The more important sections to be preserved, are 
 1. Through optic lobes, shewing : 
 
 a. The formation of the lens. 
 
 b. The involution of the primary optic vesicle. 
 
 c. The constriction, especially from above, of the 
 optic stalk. 
 
APR] EMBRYO OF THE THIRD DAY. 255 
 
 2. Through auditory sac, shewing: 
 
 a. Auditory sac still open. 
 
 b. The thin roof and thick sides of the hind-brain. 
 
 c. Notochord. 
 
 d. Heart. 
 
 e. Closed alimentary canal. 
 
 3. Through dorsal region, shewing the general appear- 
 ance of a section of an embryo at this stage, which 
 should be compared with a similar section of the 
 earlier stage. 
 
 It shews: 
 
 a. The commencement of the side folds ; the 
 alimentary canal still however open below. 
 
 b. The Wolffian duct lying close under the epiblast 
 on the outside of the pro to vertebrae. 
 
 c. The notochord with the aortae on each side. 
 
 IV. Examination of an Embryo at the end of the third 
 
 day. 
 
 A. Opening the egg, as in II. A. 
 
 B. Examination of the blastoderm in situ. 
 Observe: 
 
 1. The great increase of the vascular area both in size 
 and distinctness. The circulation is now better 
 seen in situ than after the blastoderm has been 
 removed. 
 
 2. That the embryo now lies completely on its left 
 side and that it is only connected with the yolk-sac 
 by a somewhat broad stalk. 
 
 C. Removal of the embryo. See II. C. 
 
 It is now unnecessary to remove the whole of the 
 blastoderm with the embryo; indeed it is better to 
 
256 PRACTICAL DIRECTIONS. [APP. 
 
 cut away the vascular area unless it is wanted for 
 examination. 
 
 D. S^t,rface view of the transparent embryo. 
 
 Since the embryo now lies on its side we shall 
 not have to speak of the view from above and below. 
 The views from the two sides differ chiefly as to the 
 appearance of the heart. 
 
 The embryo {freed from the blastoderm and the 
 amnion) is to be floated on to a glass slide in the 
 usual way. It is necessary to protect it while under 
 examination, with a cover-slip, which must not be 
 allowed to compress it. To avoid this, we have found 
 it a good plan to support the cover-slip at one end 
 only, since by moving it about when thus supported, 
 a greater or less amount of pressure can be applied 
 at will to the object. 
 
 The details which can at this stage be seen in a 
 transparent embryo are very numerous arid we re- 
 commend the student to try and verify everything 
 shewn in Fig. 24. Amongst the more important and 
 obvious points to be noticed are 
 
 1. The increase of the cranial flexure and the lody- 
 flexure. 
 
 2. The condition of the brain. The mid-brain now 
 forms the most anterior point of the head. 
 
 The fore-brain consists of the inconspicuous 
 vesicle of the third ventricle and the two large 
 cerebral lobes. 
 
 The hind-brain consists of a front portion, the 
 cerebellum with a thickened roof; and a hinder 
 portion, the fourth ventricle with a very thin and 
 delicate roof. 
 
 3. Organs of sense. 
 
 The eye especially is now in a very good state 
 to observe. The student may refer to Fig. 31, 
 and the description there given. . 
 
rP.] EMBRYO AT THE END OF THE THIRD DAY. 257 
 
 The ear-vesicle will be seen either just closing 
 or completely closed. 
 
 4. In the region of the heart attention must also be 
 paid to: 
 
 a. The visceral clefts. 
 
 b. The investing-mass, i.e. the growth of mesoblast 
 taking place around the end of the notochord, 
 
 c. The condition of the heart. 
 
 5. In the region of the body the chief points to be 
 observed are: 
 
 a. The increase in the number of the protovertebrcv. 
 
 b. The Wolffian duct, which can be seen as a streak 
 along the outer side of the hinder protovertebra3. 
 
 c. The allantois, which is now a small vesicle lying 
 between the folds of the somatopleure -and 
 splanchnopleure at the hind end of the body, but 
 as yet hardly projects beyond the body cavity. 
 
 F. The embryo as an opaque object. 
 Preparation as in II. F. 
 
 The general form of the embryo can be very satis- 
 factorily seen when it is hardened and examined as 
 an opaque object ; but the most important points to be 
 made out at this stage in the hardened specimens are 
 those connected with the visceral clefts and folds and 
 the mouth. 
 
 If the amnion has not been removed it will be 
 necessary to pick it completely away with needles. 
 Without further preparation a view of the visceral 
 folds and clefts may be obtained from the side; but a 
 far more instructive view is that from below, in order 
 to gain which the following method may be adopted. 
 
 Pour a small quantity of melted black wax (made 
 by mixing together lampblack and melted wax) into 
 a watch-glass, using just enough to cover the bottom 
 of the glass. While still soft make a small depression 
 in the wax with the rounded end of a pen-holder or 
 handle of a paint-brush and allow the wax to cool. 
 v 17 
 
258 PRACTICAL DIRECTIONS. [APP. 
 
 In the meantime cut off the head of the hardened 
 embryo by a sharp clean transverse incision carried 
 just behind the visceral clefts, transfer it to the watch- 
 glass and cover it with water or spirit. By a little 
 manipulation the head of the embryo may now be 
 shifted into the small depression in the wax, and thus 
 be made to assume any required position. It should 
 then be examined with a simple lens under a strong 
 reflected light, and a drawing made of it. 
 
 When the head is placed in the proper position, 
 the following points may easily be seen. 
 
 1. The opening of the mouth bounded below by the 
 first pair of visceral folds, and commencing to be 
 enclosed above by the now very small buds which 
 are the rudiments of the superior maxillary pro- 
 cesses. Compare Fig. 38. 
 
 2. The second and third visceral arches and clefts. 
 
 3. The nasal pits. 
 
 G. Sections. Manipulation as in II. G. 
 The embryo can still be stained as a whole. 
 The most important sections are : 
 
 1. Through the eyes in the three planes, vide Fig. 
 30, D. E. F. 
 
 2. Through the auditory sac. 
 
 3. Through the dorsal region, shewing the general 
 changes which have taken place. 
 
 Amongst these, notice 
 
 a. The changes of the protovertebrce : the commencing 
 formation of the muscle-plates. 
 
 b. The position of the Wolffian duct and the forma- 
 tion of the germinal epithelium. 
 
 c. The aortai and the cardinal veins. 
 
 d. The great increase in depth and relative diminu- 
 tion in breadth of the section. 
 
APP.] EMBRYO OF THE FOURTH DAY. 259 
 
 V. Examination of an Embryo of the Fourth Day, 
 
 A. Opening the egg, as in II. A. 
 
 Great care will be required not to injure the blas- 
 toderm, which now lies close to the shell-membrane. 
 
 B. Examination in situ. Observe : 
 
 1. The now conspicuous amnion. 
 
 2. The allantois, a small, and as yet hardly vascular 
 vesicle, beginning to project from the embryo into 
 the space between the true and the false amnion. 
 
 3. The rapidly narrowing somatic stalk. 
 
 C. Removal of the embryo, as in II. C. and IV. C. 
 
 The remarks made in the latter place apply with 
 still greater force to an embryo of the fourth and 
 succeeding days. 
 
 D. S^crface view of the transparent embryo. For 
 manipulation, vide IV. D. 
 
 The points to be observed are : 
 
 1. The formation of the fifth, seventh, and ninth cranial 
 nerves. 
 
 To observe these, a small amount of pressure is 
 advantageous. 
 
 2. The formation of the fourth viscefal cleft, and the 
 increase in size of the superior maxillary process. 
 
 3. The formation of the nasal pits and grooves. 
 
 4. The great relative growth of the cerebral lobes and 
 the formation of the pineal gland from the roof 
 of the vesicle of the third ventricle. 
 
 5. The great increase in the investing mass. 
 
 6. The formation and growth of the muscle-plates, 
 which can now be easily seen from the exterior. 
 
 7. The allantois. Make out its position and mode of 
 opening into the alimentary canal. 
 
260 PRACTICAL DIRECTIONS. [APP 
 
 F. The embryo as an opaqiie object. Manipulation 
 as II. F. For mode of examination vide 
 IV. F. 
 
 The view of the mouth from underneath, shewing 
 the nasal pit and grooves, the superior and inferior 
 maxillary processes and the other visceral folds and 
 clefts, is very instructive at this stage. Compare 
 Fig. 48. 
 
 G. Sections. Manipulation as in II. G. 
 
 A slightly stronger solution of chromic acid may 
 be used than for the younger embryos. 
 
 The student will most probably find that he can 
 still stain the embryo as a whole, especially if he 
 employs hsernatoxylin. If this cannot be done, the 
 sections must be stained individually after being cut. 
 The most important sections are, 
 
 1. Through the eyes. 
 
 2. Transverse section immediately behind the visceral 
 arches, shewing the origin of the lungs. 
 
 3. Transverse section just in front of the umbilical 
 stalk, shewing the origin of the liver. 
 
 4. Transverse section at about the centre of the dorsal 
 region, to shew the general features of the fourth 
 day. Compare Fig. 47. 
 
 Amongst the points to be noticed in this section, are 
 
 a. Muscle-plates. 
 
 b. Spinal nerves and ganglia. 
 
 c. "Wolffian duct and bodies. 
 
 d. Miiller's duct. 
 
 e. Mesentery. 
 
 f. Commencing changes in the spinal cord. 
 
 5. Section passing through the opening of the allantois 
 into the alimentary canal. 
 
 For the points to be observed in embryos of the 
 fifth and sixth days, the student must consult the 
 chanters devoted to those davs. 
 
IPP.] BLASTODERM OF TWENTY HO UBS. 261 
 
 In the hardened specimens, especial attention 
 should be paid to the changes which take place in 
 the parts forming the boundaries of the mouth. 
 
 In making sections, it will probably be found 
 impossible to stain the embryo as a whole; in 
 this case, the individual sections must be stained 
 separately. 
 
 VI. Examination of a Blastoderm of 20 hours. 
 
 A. Opening the egg, as in II. A. 
 
 B. Examination in situ. 
 
 It will not be found possible to make out anything 
 very satisfactory from the examination of a blasto- 
 derm in situ at this age. The student will however 
 not fail to notice the halones, which can be seen 
 forming concentric rings round the blastoderm. 
 
 C. Removal of the embryo. 
 
 Two methods of hardening can be adopted at 
 this age. One of these involves the removal of the 
 blastoderm from the yolk, as in II. C. In the other 
 case, the yolk is hardened as a whole. The latter 
 of these is the most satisfactory method for sections ; 
 but if employed, the embryo cannot be viewed as a 
 transparent object. 
 
 In the cases where the blastoderm is removed 
 from the yolk, the manipulation is similar to that 
 described under II. C, with the exception of more 
 care being required in freeing the blastoderm from 
 the vitelline membrane. 
 
 D. Surface view transparent, from above. 
 Observe : 
 
 1. The medullary groove between the two medullary 
 folds, whose hind ends diverge to enclose between 
 them the end of the primitive groove. 
 
 2. The head-fold at the end of the medullary groove. 
 
 3. The one or two pairs of protovertebrce flanking 
 the medullary groove. 
 
262 PRACTICAL DIRECTIONS. [APR 
 
 4. The notochord as an opaque streak along the floor 
 of the medullary groove. 
 
 E. Surface view transparent, from below. 
 
 Same points to be seen as from above, but less 
 clearly. 
 
 F. Embryo as an opaque object. 
 
 As an opaque object, whether the embryo is hard- 
 ened in situ or after being removed from the yolk, 
 the same points are to be seen as when it is viewed 
 as a transparent object, with the exception of the 
 notochord and protovertebrse (vide D). The various 
 grooves and folds are however seen with far greater 
 clearness. 
 
 G. Sections. 
 
 Two methods of hardening may be employed; 
 (1) with the embryo in situ, (2) after it has been 
 removed. 
 
 To harden the blastoderm in situ the yolk must 
 be hardened as a whole. After opening the egg either 
 leave the yolk in the egg-shell or pour it out into a 
 Berlin capsule; in any case freeing it as much as 
 possible from the white, and taking especial care to 
 remove the more adherent layer of white which im- 
 mediately surrounds the yolk. 
 
 Place it in a weak solution of chromic acid (first 
 of 1 p.c. and then of *5 p.c.) with the blastoderm upper- 
 most and leave it in that position for two or three days. 
 
 Care must be taken that the yolk does not roll 
 about ; the blastoderm must not be allowed to alter 
 its position: otherwise it may be hard to find it when 
 everything has become opaque. If at the end of the 
 second day the blastoderm is not sufficiently hard, 
 the strength of the solution should be increased and 
 the specimen left in it for another day. 
 
 After it has become hardened by the chromic acid, 
 the yolk should be washed with water and treated 
 successively with weak and strong spirit, vide II. G. 
 After it has been in the strong spirit (90 p. c.) for two 
 days, the vitelline membrane may be safely peeled off 
 
APP.] HAKDENING THE WHOLE YOLK. 263 
 
 and the blastoderm and embryo will be found in 
 situ. The portion of the yolk containing them must 
 then be sliced off with a sharp razor, and placed in 
 absolute alcohol. 
 
 The staining, &c. may be effected in the ordinary 
 way. 
 
 If osmic acid, which we believe will be found 
 especially serviceable for these early stages, is em- 
 ployed, it will be necessary to remove the blastoderm 
 from the yolk before treating it with the reagent. 
 
 The following transverse sections are the most im- 
 portant at this stage : 
 
 1. Through the medullary groove, shewing 
 
 a. The medullary folds with the thickened meso- 
 blast. 
 
 b. The notochord under the medullary groove. 
 
 c. The commencing cleavage of the mesoblast. 
 
 2. A section through the region where the medullary 
 folds diverge, to enclose the end of the primitive 
 groove, shewing the greatly increased width of the 
 medullary groove, but otherwise no real alteration 
 in the arrangement of the parts. 
 
 3. A section through the front end of the primitive 
 groove with the so-called axis cord underneath it, 
 while on each side of it are still to be seen the 
 medullary folds. 
 
 4. A section through the primitive groove behind this 
 point, shewing the typical characters of the primitive 
 groove. 
 
 VII. Examination of an unincubated Blastoderm. 
 
 A. Opening the egg. Vide II. A. 
 
 B. Examination of the blastoderm in situ. 
 
 Observe the central white spot and the peripheral 
 more transparent portion of the blastoderm and the 
 halones around it. 
 
264< PRACTICAL DIRECTIONS. [APP. 
 
 C. Removal of the blastoderm. Vide VI. C. 
 
 With the unincubated blastoderm still greater care 
 is required in removal than with the 20 hours' blasto- 
 derm, and there is no special advantage in doing so 
 unless it is intended to harden it with osmic acid. 
 
 D. Surface view transparent from above. 
 Observe the absence of the central opacity. 
 
 E. Sitrface view transparent from underneath. 
 Nothing further to be observed than from above. 
 
 F. As an opaque object. 
 
 There is nothing to be learnt from this. 
 
 G. Sections. 
 Manipulation as in VI. G. 
 
 Only one section is required, viz. one taken through 
 the centre of the blastoderm, shewing: 
 
 a. The distinct epiblast. 
 
 b. The lower layer cells not as yet differentiated 
 into mesoblast and hypoblast. 
 
 c. The thickened edge of the blastoderm. 
 
 d. The segmentation cavity and formative cells. 
 
 VIII. Examination of the process of Segmentation. 
 
 To observe the process of segmentation it will be 
 found necessary to kill a number of hens which arc 
 laying regularly. The best hens lay once every 24< 
 hours, and by observing the time they usually lay (and 
 they generally lay pretty regularly about the same 
 time), a fair guess may be made beforehand as to 
 the time the egg has been in the oviduct. By this 
 means a series of eggs at the various stages of seg- 
 mentation, may usually be obtained without a great 
 unnecessary sacrifice of hens. For making sections, 
 the yolk must in all cases be hardened as a whole, 
 which may be done as recommended in VI. G. 
 
APP.] SEGMENTATION. 2G5 
 
 Chromic acid is an excellent reagent for this and it 
 will be found very easy to make good sections. 
 
 In the sections especial attention should be paid, 
 
 1. To the first appearance of nuclei in the segments, 
 and their character. 
 
 2. To the appearance of the horizontal furrows. 
 
 3. As to whether new segments continue to be formed 
 outside the limits of the germinal disc, or whether 
 the fresh segmentation merely concerns the already 
 formed segments. 
 
 4. In the later stages, to the smaller central and larger 
 peripheral segments, both containing nuclei. 
 
 For surface views, the germinal disc, either fresh 
 or after it has been* hardened, can be used. In 
 both cases it should be examined by a strong 
 reflected light. The chief point to be noticed is the 
 more rapid segmentation of the central than of the 
 peripheral spheres. 
 
 IX. Examination of the later changes of the Embryo. 
 
 For the later stages, and especially for the develop- 
 ment of the skull and the vascular system of the body 
 of the chick, it will be found necessary to dissect the 
 embryo. This can be done either with the fresh 
 embryo or more advantageously with embryos which 
 have been preserved in spirit. 
 
 If the embryos are placed while still living into 
 spirit a natural injection may be obtained. And such 
 an injection is the best for following out the arrange- 
 ment of the blood-vessels. 
 
 Sections of course will be available for study, 
 especially when combined with dissections. 
 
 X. Study of the development of the Blood-vessels. 
 
 Observations on this subject must be made with 
 blastoderms of between 30 40 hours. These are to 
 be removed from the egg, in the usual way (vide II. A. 
 and C.) spread out over a glass slip and examined 
 from below, vide II. E. 
 
266 PRACTICAL DIRECTIONS. [APP. 
 
 The blastoderm when under examination must be 
 protected by a cover-slip with the usual precautions 
 against pressure and evaporation, and a hot stage 
 must also be employed. 
 
 Fresh objects so prepared require to be examined 
 with a considerable magnifying power (400 to 800 
 diameters). From a series of specimens between 30 
 and 40 hours old all the points we have mentioned in 
 Chapter IV. 6 can without much difficulty be observed., 
 
 Especial attention should be paid in the earlier 
 specimens to the masses of nuclei enveloped in proto- 
 plasm and connected with each other by protoplasmic 
 processes ; and in the later stages to the conversion of 
 these nuclei into blood corpuscles and of the proto- 
 plasmic processes into capillaries, with cellular walls. 
 
 Blastoderms treated in the following ways may be 
 used to corroborate the observations made on the 
 fresh ones. 
 
 1. With gold chloride. 
 
 Immerse the blastoderm in gold chloride (*5 p. c.) 
 for one minute and then wash with distilled water 
 and mount in glycerine and examine. 
 
 By this method of preparation, the nuclei and 
 protoplasmic processes are rendered more distinct, 
 without the whole being rendered too opaque for 
 observation. 
 
 The blastoderm after the application of the gold 
 chloride should become a pale straw colour ; if it 
 becomes in the least purple, the reagent has been 
 applied for too long a time. 
 
 2. With potassium bichromate. 
 
 Immerse in a 1 p. c. .solution for one day and then 
 mount in glycerine. 
 
 3. With osmic acid. 
 
 Immerse in a "5 p. c. solution for half an hour and 
 then in absolute alcohol for a day, and finally 
 mount in glycerine. 
 
INDEX. 
 
 A. 
 
 AFANASSIEFP, 64, 71, 72. 
 
 Air-chamber, 12. 
 
 Albumen, 13. 
 
 Aliethmoid cartilage, 236. 
 
 Alimentary canal, 61. 
 
 Alinasal cartilages, 227, 231, 236. 
 
 Aliseptal cartilages, 231, 236. 
 
 Alisphenoid, 232. 
 
 Allan toic arteries, 168. 
 
 Allantoic veins, 210. 
 
 Allantois, 41, 147150, 174, 201, 
 
 202, 203. 
 Amnion, 3941, 57> 8 3 H 1 * 2Or , 
 
 202 ; cavity of, 40. 
 Amniotic fluid, 41 ; sac, 41. 
 Annuli fibrosi, 158. 
 Anterior fissure, 186, 187. 
 Anus, 183. 
 Aorta, dorsal, 121 ; primitive aortse, 
 
 66, 79. 
 
 Aortic arches, Si, 121, 213, 217, 218. 
 Aquapendente, FABRICIUS of, 2. 
 Aqueductus vestibuli, in, 115. 
 Area opaca, 16, 18; pellucida, 16. 
 AEISTOTLE, 2. 
 
 ArteriaB collaterals colli, 217. 
 Arterial arches, 168, 169; arterial 
 
 system, 213 220. 
 Arteries; allantoic 168; iliac 168, 
 
 214 ; omphalo-mesaraic 66, 80, 168, 
 
 202, 214; subclavian 217; umbilical 
 
 1 68 ; vertebral 217. 
 Auricles, 63, 79, 173. 
 Axis-cord, 48. 
 
 B. 
 
 BABUCHIN, 106. 
 BAER. See VON BAER. 
 Basibranchial, 230. 
 Basi-hyal, 230. 
 
 Basioccipital, 236. 
 
 Basipterygoid, 236. 
 
 Basisphenoid, 231. 
 
 Basisphenoidal ossicles, 235. 
 
 Basitemporals, 232, 235, '236. 
 
 Beak, 205. 
 
 Blastoderm, 5, 14, 16 18, 174, 241, 
 242, 252 ; of 20 hours, 261 ; uniii- 
 cubated 263. 
 
 Blood-vessels, 66 72 ; study of de- 
 velopment of, 265, 266. 
 
 Body-folds, 31. 
 
 BOETTCHER, 113, 114, Il6. 
 
 Bone; metatarsal 176; occipital 227 ; 
 
 tarsal, 176. 
 BONNET, 4. 
 BORNHAUPT, 149, 162. 
 Branchial clefts, 119. 
 Branchial fold, 119. 
 Bulbus arteriosus, 63, 79, 123, 173, 
 
 190194, 214, 215. 
 
 C. 
 
 Canal; alimentary 61; semicircular, 
 115. 
 
 Canales Botalli, 219, 220. 
 
 Canalis auricularis, 1 23, 1 72 ; reuniens, 
 in. 
 
 Capsules, periotic, 227. 
 
 Cardinal veins, 124. 
 
 Carotids, 169, 213. 
 
 Carpals, 175, 176. 
 
 Carpo-metacarpus, 175. 
 
 Carpus, 175. 
 
 Cartilage, 230 232 ; aliethmoid 236 ; 
 alinasal227, 231, 236; aliseptal 231, 
 236; ethmoido-nasal 236; ethmo- 
 presphenoid 235, 236; Meckel's, 
 179, 182, 229, 230; paired rods of, 
 227; pre-nasal, 231, 232. 
 
268 
 
 INDEX. 
 
 Cartilage-bone, 178. 
 
 Cartilaginous rods, 228; skeleton, 
 
 206 ; skull, 230 233. 
 Cavity of the amnion, 40. 
 Cell-mass, intermediate, 82, 136, 161, 
 
 165. 
 
 Centrale, 176. 
 Cerato-branchial, 230. 
 Cerato-hyals, 230. 
 Cerebellum, 93. 
 Cerebral vesicles, 58 60. 
 Chalazee, 13. 
 
 Change of position of embryo, 87. 
 Chorion, 32, 42, 203. 
 Choroid, 102. 
 
 Choroidal fissure, 99 101. 
 Cicatricula, 14. See Blasto'derm. 
 Circulation, 79, 81, 168173, 220 
 
 223. 
 CLARKE, Lockhart, 153, 185, 187; 
 
 L. C., 186. 
 Cleavage of the mesoblast, 38, 57, 65, 
 
 202, 203. 
 Clefts; branchial, 119; visceral, 119, 
 
 204. 
 
 Clinoid walls 231. 
 Cochlea, in, 112. 
 Condyle, occipital, 230, 233, 236. 
 Cones, 106. 
 Coni vasculosi, 167. 
 Corpora bigemina, 93. 
 CORTI, rods of, 117. 
 COSTE, 22, i8r. 
 Cranial flexure, 78, 87, 89, 143; 
 
 cranial nerves, 137, 138, 147. 
 Cranium, 177 179, 225 238. 
 Crura cerebii, 93. 
 Curvature of the body, 87, 
 Cutting sections, 250 252. 
 
 D. 
 
 DARESTE, 15. 
 
 DESCEMET, membrane of, 102. 
 
 Descent of the ovum, 21. 
 
 Digits, 175. 
 
 Disc, germinal, 19. 
 
 Discus proligerus, 19. 
 
 DOBRYNIN, 125, 148, 149. 
 
 DoLLINGER, 5. 
 
 Dorking fowls, 176. 
 
 Dorsal aorta, 121. 
 
 Duct of Miiller, 162, 163, 168; um- 
 bilical duct, 143, 174. See Wolffian 
 duct. 
 
 Ductus arteriosi, 219; Botalli, 219; 
 cochlearis, in, 115; Cuvieri, 124, 
 I7i;venosus, 123, 170, 210. 
 
 Duodenum, 128. 
 
 DURSY, 50, 78, 82, 91, 156. 
 
 E. 
 
 Ear, 78, in; 117, 237. 
 
 Ectosteai ossification, 233, 237. 
 
 Ectostosis, 233. 
 
 Egg-shell, n. 
 
 Elbow, 175. 
 
 Embryo as an opaque object, 245, 
 
 2 54> 2 57> 2 6o, 262; of the third 
 
 day, examination of, 252 258; of 
 
 the fourth day, 259 261. 
 Embryology, meaning of, i. 
 Embryonic sac, 35; shield, 44. 
 Endostosis, 233. 
 Epiblast, 27, 44, 56, 197. 
 Epididymis, 167. 
 Epigenesis, 3. 
 Epiotic, 238. 
 Episkeletal muscles, 159. 
 Epithelium, germinal, 160, 165. 
 Ethmoid, 237. 
 
 Ethmoido -nasal cartilage, 236. 
 Ethmo-presphenoid cartilage, 235, 
 
 236; plate, 231. 
 Eustachian valve, 195. 
 Evolution, 3. 
 Examination of the blastoderm in 
 
 situ, 241, 242, 252, 255. 
 Exoccipital plates, 236. 
 Exoccipitals, 230. 
 Eye, 117. 
 Eyeball, 94. 
 
 F. 
 
 FABRICIUS, 2, 3. 
 
 Feathers, 205. 
 
 Fenestra ovale, 232; rotund um, 232. 
 
 Fenestrae, 236. 
 
 Fibulare, 176. 
 
 Fissure; anterior, 186, 187 ; choroidal, 
 
 99 101 ; posterior, 187. 
 Foetal appendages, 201. 
 Foot, 175, 176. 
 
 Foramen ovale, 194; occipital, 227. 
 Fore-brain, 78, 91. 
 Foregut, 62, 
 Formative cells, 17, 26, 44. 
 
INDEX. 
 
 2C9 
 
 Fourth ventricle, 94. 
 Fretum Halleri, 172. 
 F rentals, 235. 
 Fronto-nasal process, 120, 145. 
 
 G. 
 
 Gall-bladder, 133. 
 
 (^asserian ganglion, 147. 
 
 GEGENBAUR, 154, 157, 158, 228, 229. 
 
 Genital ridge, 164. 
 
 Germinal disc, 19 ; epithelium, 16.0, 
 
 165; spot, 20; vesicle, 20. 
 Gland, pineal, 91. 
 Glossopharyngeal nerve, 147. 
 GOETTE, 25, {,0, 72, 93, 128, 129, 131, 
 
 J 33, H7. 
 
 Graffian follicle, 166, 167. 
 Grey column, 184, 185 ; grey matter, 
 
 184, 185, 186, 187, 189. 
 
 H. 
 
 Hcematoxylin, solution of. 247, 248. 
 
 Halones, 52. 
 
 HALLER, 4, 5. 
 
 Hand, 175, 176. 
 
 Hardening, 246, 247. 
 
 HARVEY, 2, 3. 
 
 HASSE, 112, 155. 
 
 Head, 145. 
 
 Head-fold, 29, 33, 34, 4 8, 57. 
 
 Heart, 6366, 122, 123, 172, 173, 
 
 190 196. 
 Hen's egg, structure of, TI 19; 
 
 changes in, before it is laid, 19 26. 
 HEN SEN, 82. 
 Hepatic veins, 210. 
 HEEING, 133. 
 Hind -brain, 78, 93. 
 His, 16, 48, 52, 55, 64, 68, 71, 72, 
 
 82, 83, 124, 134, 149, 153, 156, 
 
 159, 168. 
 
 HOPPE-SEYLER, 15. 
 HUXLEY, 76, 90, 137, 159, 179, 181, 
 
 182, 228. 
 
 Hypoblast, 27, 45,^51, 56, 198. 
 Hypophysis cerebri, 91. 
 
 I. 
 
 Iliac arteries, 168, 214. 
 Imbedding, 248 250. 
 Incubation, 2742. 
 
 Incubators, 239, 240. 
 Infundibulum, 91, 92. 
 Intermediate cell-mass, 82, 136, 161, 
 
 165. 
 
 Intermedium, 175, 176. 
 Investing mass, 177, 178, 225 227. 
 Iter a tertio ad quartum ventriculum, 
 
 93- 
 
 J. 
 
 JAEGER, 158. 
 Jugal, 235. 
 Jugular vein, 207. 
 
 K. 
 
 KAPFF, 167. 
 
 Kidneys, permanent, 163, 164. 
 KLEIN, 65, 72. 
 
 KLEINENBERG, 246, 248, 249. 
 Knee, 175. 
 
 KOELLIKER, 6, 71, 82, xor, ioS, 133, 
 134, 159, 181. 
 
 KUPITER, 163. 
 
 L. 
 
 Lacrymals, 237. 
 
 Laminae dorsalts, 48. 
 
 Lateral column, 186, 187. 
 
 Lateral folds, 76; plate, 54; lateral 
 
 ventricle, 91. 
 Lens, 98, 107, 108. 
 Lens-capsule, ioS, 109. 
 LEOPOLD, 167. 
 
 LlEBERKUEHN, 98, IOI, IO2, IO7, ioS. 
 
 Ligamentum suspansoriurn, 158. 
 
 Limbs, 143, 145, 174, 175. 
 
 Liquor amnii, 41. 
 
 Liver, 131 133. 
 
 Lungs, 129 131. 
 
 Lung- vesicle, primary, 131. 
 
 M. 
 
 MALPIGHI, 3. 
 
 Malpighian bodies, 164. 
 
 Mandibular arch, 230. 
 
 Manus, 175, 176. 
 
 Marginal process, 233. 
 
 Maxillae, 120. 
 
 Maxillares, 233. 
 
 Maxillary processes, 179,229. 
 
 Maxillo-palatine, 235. 
 
 Meatus venosus, 123, 169, 209. 
 
270 
 
 INDEX. 
 
 MECKEL'S cartilage, 179, 182, 229, 
 
 230. 
 
 Medulla oblongata, 93. 
 Medullary canal, 53; folds, 37, 48, 
 
 57; groove, 48, 57. 
 Membrana limitans externa, 106. 
 Membrane of Descemet, 102 ; of 
 
 Reissner, 112; vitelline membrane 
 
 13, 20. 
 
 Membrane-bones, 178, 233, 237, 238. 
 Membranous labyrinth, 1 1 1 113. 
 Meniscus, 158. 
 Mesenteric veins, 171, 210. 
 Mesentery, 127. 
 
 Mesoblast, 27, 45, 56, 196, 198, 199. 
 Metacarpals, 175, 176. 
 Metatarsal bones, 176. 
 Metatarsus, 176. 
 Mid-brain, 78, 93, 145* 
 MIESCHER, 15. 
 Mouth, 179, .182, 183. 
 Movements of the embryo, 223, 224. 
 Mucous layer, 45. 
 MUELLER, 92, 93, 133, 134, 156, 157, 
 
 228; duct of M. 144; M.'s duct, 
 
 126, 162 ; fibres of M., 107. 
 Muscles, episkeletal, 159. 
 Muscle-plates, 135, 136, 159. 
 
 N. 
 
 Nails, 205. 
 
 Nasal labyrinth, 181 ; pits, 117, 145; 
 
 processes, 180, 233. 
 Nasals, 235. 
 NATHUSIUS, ir. 
 Neck, 119. 
 Nerves; cranial, 137, 138, 147; 
 
 glossopharyngeal, 147; optic, 107; 
 
 pneumogastric, 147. 
 Neural canal, 53; tube, 37. 
 Notochord, 49, 57, 78, 156, 157. 
 Nucleus of Pander, 15; nucleus pul- 
 
 posus, 158. 
 
 O. 
 
 Occipital bone, 227. 
 
 Occipital condyle, 230, 233, 236. 
 
 Occipital foramen, 227. 
 
 OELLACHER, 20, 21, 22, 25. 
 
 (Esophagus, 127. 
 
 Olfactory vesicle, 117. 
 
 Omphalo-mesaraic arteries, 66, 80, 
 
 168, 202, 214; veins, 63, 69, 80, 
 
 170, 202. 
 
 Opening the egg, 240, 241. 
 
 Opisthotic, 237. 
 
 Optic cup, 97, 103; nerve, 107; 
 
 vesicles, 76, 77, 94, 95, 96. 
 Osseous labyrinth, in 114. 
 Ossification, 206; of the cranium, 233, 
 
 235 ; of the ear, 237. 
 Otic vesicle, in. 
 Ovarian ovum, 19. 
 Oviduct, 20, 1 68. 
 Ovum, descent of the, 21 ; primordial 
 
 ova, 1 66, 167. 
 
 P. 
 
 Paired rods of cartilage, 227. 
 
 Palatine, 233. 
 
 Palatine rod, 230, 232. 
 
 Pancreas, 133. 
 
 PANDER, 5,71; nucleus of P., 15, 17. 
 
 Parasphenoid, 235. 
 
 Parietals, 235. 
 
 PARKER, 177, 179, 225, 226, 228, 229, 
 
 231, 232, 234, 236. 
 Parostosis, 233. 
 Parovarium, 167. 
 Pecten, 105. 
 
 Pellucid area, 57. See Area pellucida. 
 PEREMESCHKO, 133. 
 Pericardium, 194. 
 Periotic capsules, 227. 
 Pes, 176. 
 PFLUEGER, 167. 
 Phalaijges, 176. 
 Pineal gland, 91. 
 Pituitary body, 91, 92, 227; di- 
 
 verticulum, 92, 93; space, 178, 
 
 227, 232. 
 
 Pleuroperitoneal cavity, 38, 54. 
 Pneumogastric nerve, 147. 
 Portal vein, 210. 
 Posterior fissure, 187. 
 Post-frontal, 237. 
 Premaxillaries, 233, 237. 
 Prenasal cartilage, 231, 232. 
 Preservation of the embryo as a whole, 
 
 252. 
 
 Primary lung-vesicle, 131. 
 Primitive aortse, 66, 79; groove, 47, 
 
 57; streak, 46, 47, 50. 
 Primordial ova, 166, 167. 
 Prootic, 237. 
 Proto vertebrae, 55, 57, 134, 151 
 
 159- 
 
 Pterotic, 237. 
 
INDEX. 
 
 271 
 
 Pterygoid rod, 230, 232. 
 Pterygo- palatine rod, 179. 
 Punctum saiieus, 2. 
 PURKINJE, 6. 
 
 Q. 
 
 Quadrate, 179, 230, 232, 236. 
 Quadrato-j ugals, 235. 
 
 Radiale, 175. 
 
 RATHKE, 6, 91, 92, 177, 215, 216, 
 
 217, 225, 228. 
 
 Recessus vestibuli, m, 115. 
 REICHERT, 91, 148. 
 REISSNER, membrane of, 112. 
 REMAK, 6, 64, 71, 72, 78, 82, 97, ror, 
 
 1 06, 1 08, 133, 134, 138, 147, 148, 
 
 150, 153, 163. 
 
 Removal of the embryo, 242, 261. 
 Retina, 105. 
 Ribs, 158. 
 Rods and cones, 106; rods of Corti, 
 
 117 ; paired rods of cartilage, 227. 
 ROMITI, 83, 167. 
 ROSENBERG, 175, 176. 
 Rostrum, 232, 235, 236. 
 
 S. 
 
 Sac; amniotic 41 ; embryonic 35. 
 Sacculus hemisphericus, 115. 
 SCHENK, 133, 137. 
 SCHULTZE, Max, 1 06. 
 
 SCHWARCK, 155, 157. 
 
 Sclerotic, 102. 
 
 Sections, 250252, 254, 255, 258, 260, 
 
 261, 262, 263, 264. 
 Segmentation, 22 26, 264, 265 ; se- 
 
 condary of vertebral column, 155. 
 Segmentation-cavity, 25. 
 Semicircular canals, 115. 
 Semilunar valves, 191, 19?. 
 Seminiferous tubules. See Tubuli 
 
 seminiferi. 
 Septum nasi, 237 ; septum of the 
 
 bulbus arteriosus, 215; ventricular 
 
 septum 172, 173. 
 SERNOFF, 162, 167. 
 Serous cavity, 38 ; layer, 45. 
 Sexual eminence, 166. 
 Shell- membrane, 12. 
 Sinus rhomboidalis, 60, 190; termi- 
 
 nalis, 69; venosus, 123, 170. 
 
 Skull, 177179, 225238. 
 
 SMIDT, 91. 
 
 Somatic stalk, 39, 143. 
 
 Somatopleure, 38, 54, 57. 
 
 Spinal cord, 183 190; ganglia, 152, 
 
 153. 
 
 Splanchnic stalk, 39, 141, 174. 
 
 Splanchnopleure, 38, 54, 57. 
 
 Spleen, 133. 
 
 Splint-bones, 233, 238. 
 
 Splitting of the mesoblast. See Cleav- 
 age. 
 
 Squamosal, 235. 
 
 Staining, 247. 
 
 Stapes, 232. 
 
 Stomach, 128. 
 
 STRICKER, 133. 
 
 Stroina, 166. 
 
 Subclavian arteries, 217. 
 
 Summary of changes during the first 
 day, 56, 57; second day, 83; third 
 day, 139, 140; fourth day, 173; 
 fifth day, 199 
 
 Supraoccipital plates, 236. 
 
 Supraoccipitals, 232, 233. 
 
 Surface view of embryo, 243, 244, 
 253> 256, 259, 262. 
 
 Sutures, 238. 
 
 T. 
 
 Tail, 125, 143. 
 
 Tail-fold, 35, 75, 125, 143. 
 
 Tarsal bones, 176. 
 
 Tarso-metatarsus, 176. 
 
 Testes, 167. 
 
 THOMSON, Allen, 6, 12. 
 
 Thyroid body, 133, 134. 
 
 Tibiale, 176. 
 
 TONGE, 190, 191, 215. 
 
 Tongue, 205. 
 
 Trabeculae cranii, 178, 179,227, 228. 
 
 Trachea, 131. . 
 
 Tread, 2. 
 
 Tubuli seminiferi, 167. 
 
 Tubuli uriniferi, 164: 
 
 'Tween brain, 91. 
 
 U. 
 
 Ulnare, 175. 
 
 Umbilical arteries, i53; duct, 143, 
 
 174; veins, 172, 210. 
 Ureter, 163. 
 Utriculus, 115. 
 
INDEX. 
 
 V. 
 
 Vas deferens, 168. 
 
 Vascular area, 28, 56, 57, 84, 85; v. 
 layer, 45. 
 
 Veins; allantoic 210; cardinal 124; 
 of the liver, 171; hepatic 210; 
 jugular 207; mesenteric 171, 210; 
 omphalo-mesaraic 63, 69, 80, 170, 
 202; portal 210; umbilical 172, 
 210; vena cava inferior, 171, 207, 
 209; vena terminalis, 69; venae 
 advehentes, 171, 209, 210; veme 
 cavas, 209; venae revehentes, 171, 
 209, 210. 
 
 Venous circulation, 170, 21 1 ; venous 
 system, 169 172. 
 
 Ventricles, 79, 193; fourth ventricle 
 
 94- 
 
 Ventricular septum, 172, 173. 
 Vertebral arteries, 217; column, 152 
 
 155; plate, 54, 56. 
 Vesicle, germinal, 20 ; olfactory 1 1 7 ; 
 
 vesicle of the cerebral hemispheres, 
 
 78; optic vesicle 76, 77, 94, 95, 
 
 96 ; otic vesicle in; vesicle of the 
 
 third ventricle, 91. 
 Vestibule, in. 
 Visceral arches, 179.; clefts, 119, 204; 
 
 folds, 119. 
 
 Vis essentialis, 5. 
 
 Vitellin, 15. 
 
 Vitelliiie membrane, 13, 20. 
 
 Vitreous humour, 101. 
 
 Vomer, 237. 
 
 VON BAER, 5, 6, 7, 41, 64, 65, 71, 
 72, 78, 86, 91, 148, 150, 166, 173, 
 190, 194, 201, 202, 206, 213, 214, 
 215, 217, 219. 
 
 W. 
 
 WALDEYEK, 82, 149, 160, 161, 162, 
 
 163, 165, 167. 
 White of egg, 13. 
 White columns, 184, 185, 186, 189; 
 
 white matter, 184, 185, 186, 187, 
 
 189 ;* white yolk, 15. 
 Wing, 175. 
 WOLFF, 4, 5, 6, 71. 
 Wo'.ffiaii bodies, 139, 161 163, 167; 
 
 Wolffian duct, 73, 82, 83, 138, 139, 
 
 160 162, 168; Wolffian ridge, 143. 
 
 Y. 
 
 Yolk, 14 1 6. 202; yellow yolk, 14; 
 
 white yolk, 15. 
 Yolk-sac, 35. 
 
 CAMBRIDGE: PRINTED BY c. J. CLAY, M.A. AT THE UNIVERSITY PRESS. 
 
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